PC Error 0x80070643: Fatal Error During Installation
Error 0x80070643 means 'fatal error during installation,' but fatal does not mean your PC is broken or your files are gone. It means the installer stopped and did not finish. The failed update is not installed yet, and in most cases you can fix the cause and try it again.
This code is confusing because the same message shows up in two different places. It can appear on a Microsoft Defender definition update, the regular update that keeps Windows Security current on new threats. It can also appear on a .NET update, or on a .NET or program install.
That difference decides the fix, so do not start by retrying the same item again and again. First, find the exact name of what failed. If it was Defender, follow Repair Path A below. If it was .NET, follow Repair Path B, which has different steps.
Part of the Windows Error Codes in Plain English: Look Up Any Code series, the pillar guide for this cluster.
First, Tell Which One Failed: Defender or .NET?
The code alone will not tell you which fix to use. Open Settings > Windows Update > Update history and find the failed entry for 0x80070643. Read its full name and KB number if one is listed, and write down that name, the KB number, and the date it failed before you retry anything.
If the name is "Security Intelligence Update for Microsoft Defender Antivirus," it is the Defender variant. Follow Repair Path A only, later on this page. The .NET steps will not fix a failed Defender definition update.
If the name includes ".NET Framework," or the error came from a .NET installer or a program install that ended in "fatal error," it is the .NET variant. Follow Repair Path B only. The Defender steps will not fix a failed .NET update or install.
If neither name matches, do not guess. Skip both repair paths and go to the shared fallback later on this page. For a fuller guide to reading Update history, see "Did That Update Actually Install?" Keep your note handy, because later steps ask for that exact item.
Before Either Fix: Three Safe Checks in Five Minutes
Some failures are a one-time glitch after the PC has been on for days. A full restart clears that state, so try this before either repair path. Select Start > Power > Restart, not Shut down, and wait until the desktop loads before you continue.
Next, connect to a stable Wi-Fi signal, or plug in a wired connection if you can. Then retry only the exact item you wrote down from Update history, once. If it installs this time, you are done and you do not need either repair path.
While you wait, note whether another antivirus is running, or whether a work or school account manages this PC. It does not change the .NET path, but it can change the Defender path, where Defender may not be the program in charge.
Do not download a fix from a cleaner, updater, or DLL site. Registry cleaners, driver updaters, and paid fix-it tools cannot fix this code and can add new problems. If the retry shows 0x80070643 again, stop retrying and go to your repair path.
Repair Path A: When the Defender Update Failed
Repair Path A: When the Defender Update Failed
Use this path only if the failed item was a Security Intelligence Update for Microsoft Defender Antivirus. Start by updating from inside Windows Security instead of Windows Update. Open Windows Security, select Virus & threat protection, then Protection updates, then Check for updates.
Let the check finish, then look at the Security intelligence line on the same page. Write down the version number and the date and time shown. Those details are the same definitions named in the failed update, so a new version with today's date means this first step worked.
If the check fails again, or the date does not change, clear the stale definitions that are already on the PC. This uses a removal command built into Defender. It removes the old definitions only, and you will replace them in the last step below.
- Select Start, type Command Prompt, and select Run as administrator. Select Yes if Windows asks for permission.
- Type cd C:\Program Files\Windows Defender and press Enter. This takes you to the folder where the MpCmdRun tool is stored.
- Type MpCmdRun.exe -RemoveDefinitions -All and press Enter. Wait for the command to finish before you close the window.
- Open Windows Security again and select Virus & threat protection, then Protection updates, then Check for updates. Let the fresh set of definitions download in full.
Next, check that Defender is the antivirus actually in charge. In Windows Security, select Virus & threat protection, look for Who's protecting me?, and select Manage providers. The page lists which antivirus is on and which is off.
If a third-party antivirus is on and Microsoft Defender Antivirus is off, the other program has taken over. In that case, a Defender definition failure may be expected and needs no repair. Leave the working antivirus in place.
Judge the result on the Protection updates page, not in Update history. A current security intelligence version with today's date and time means the fix worked, even if the earlier failed entry is still listed in Update history.
Repair Path B: When a .NET Update or Install Failed
Use this path if the failed item in Update history named .NET Framework, or if a program installer closed with 'fatal error during installation.' Before you change anything, write down the exact name, the KB number if one is listed, and the date. Keep that note to match a repeat failure later.
Start with Microsoft's own repair. Search Microsoft Support for '.NET Framework Repair Tool,' download it from Microsoft, and run it. Accept the license terms, click Next, and let it apply the fixes it finds.
Restart your PC when the tool finishes, even if it does not ask you to. Then retry the one item that failed, and nothing else yet. For a Windows Update item, go to Settings, then Windows Update, and select Check for updates.
If the same item fails again with 0x80070643, check that Windows has the right .NET version turned on. Open Turn Windows features on or off using the route for your version below. Look for .NET Framework 3.5 and .NET Framework 4.8 Advanced Services.
On Windows 11, go to Settings, then Apps, then Optional features, and under Related settings select More Windows features. On Windows 10, open Control Panel, then Programs, then select Turn Windows features on or off. Both routes open the same window.
If the box beside the version your update or program needs is empty, select it so it shows a check mark, then select OK and let Windows finish. Leave a box alone if it already shows a check mark. .NET Framework 3.5 includes .NET 2.0 and 3.0, and many older programs need it.
Restart your PC after Windows finishes, then retry the failed item once.
If your failure came from a standalone program installer, do the repairs above first, before you try that program again. Then download a fresh copy from the program maker's official website only, and run that copy to install it again. Do not use a download site that bundles other software.
If the same KB number or the same program fails again with 0x80070643, stop and do not keep reinstalling it. Keep your note of the name, KB number, and date, and move on to the shared fallback in the next section.
If Your Variant's Path Did Not Work: The Shared Fallback
If Path A or Path B did not fix the same failed item, try this shared fallback once. It works on the update parts both variants use. Run the Windows Update troubleshooter first: Windows 11: Settings > System > Troubleshoot > Other troubleshooters, then Run beside Windows Update. Windows 10, including ESU-enrolled PCs: Settings > Update & Security > Troubleshoot > Additional troubleshooters, then Windows Update > Run the troubleshooter.
If the troubleshooter does not fix it, reset the Windows Update cache. Open Command Prompt as administrator and enter these commands in this order:
- net stop wuauserv
- net stop bits
- ren C:\Windows\SoftwareDistribution SoftwareDistribution.old
- net start bits
- net start wuauserv
The renamed folder keeps the old copy while Windows builds a fresh one.
Restart your PC, then retry only the one Defender or .NET item that failed at the start. If 0x80070643 returns, stop and write down its exact name, KB number if listed, and the date it failed. Keep those details. A repair install is a last resort only, so follow the linked guide if you reach that point.
For deeper system damage, the linked error 0x80073712 guide covers DISM, SFC, and Reset This PC. Use that guide instead of guessing at those tools here, and do not keep repeating the steps above if the same item has failed again.
Check It Worked, and When to Stop Retrying
Check it worked, and when to stop retrying
If you followed the Defender path, open Windows Security, then Virus & threat protection, then Protection updates. A current security intelligence version with a recent date and time means Defender is up to date and the fix worked.
If you followed the .NET path, check Update history for the newest entry for the same item. If it shows as successfully installed, open the program that needed .NET. If the program opens and works normally, the fix worked.
Do not worry if the old failed entry is still listed below it. Failed entries can stay in Update history after a later success. Judge the result by the newest entry for that exact item, not by the older failure.
After your path and the shared fallback, stop reinstalling the same failing item. Do not pay a pop-up or a caller who offers to fix 0x80070643. If an update returns after a success, see 'Windows keeps installing the same update again and again.'
Conclusion
With 0x80070643, the useful clue was never the word 'fatal.' It was the name next to the code. A Defender definition and a .NET update can share the same number and still need different treatment, which is why guessing or repeating the same install tends to go nowhere.
Your PC and your files were not the problem the message described. The installer stopped, and that is a narrower, fixable problem. Work from the exact failed item, judge the result by what happens next, and stop once it succeeds.