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0x80070643

A generic "installation failed" error. In 2024 it became famous because the KB5034441 security update failed with it on PCs whose recovery partition was too small. It also appears when Defender or .NET updates fail.

How to fix it

1. Restart the PC and run Windows Update again: one-off failures often pass the second time.
2. If the failing update is KB5034441 (or another "Windows Recovery Environment" update), the cause is a recovery partition that is too small. Microsoft's fix is to enlarge it by about 250 MB, following its article KB5028997. Back up first, since it involves editing partitions.
3. For other updates, reset Windows Update (see the steps below).
4. Open Command Prompt as administrator.
5. Stop the update services: net stop wuauserv, then net stop cryptSvc, then net stop bits, then net stop msiserver
6. Rename the update caches: ren C:\Windows\SoftwareDistribution SoftwareDistribution.old and ren C:\Windows\System32\catroot2 catroot2.old
7. Start the services again: net start wuauserv, net start cryptSvc, net start bits, net start msiserver
8. Restart the PC and run Windows Update again.
9. If one update keeps failing, download it by its KB number from the Microsoft Update Catalog (catalog.update.microsoft.com) and install it by double-clicking the file.

Is it serious?

Your PC is safe to use meanwhile. For KB5034441, many people leave it uninstalled; the partition fix is only worth doing if you are comfortable with disk tools.