How do you check for and fix corrupt registry entries related to Windows Update error 0x80070643?

Make a registry backup using Regedit before attempting to examine and repair faulty registry entries causing Windows Update Error 0x80070643. Next, check for missing or corrupt entries in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate manually or with a registry cleaner like CCleaner. The following commands can also be used to reset Windows Update registry keys in the Command Prompt (Admin):

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate /v AccountDomainSid /f

reg delete HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate /v PingID /f

reg delete HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate /v SusClientId /f

After this, restart your PC and check for updates again. If the issue persists, try SFC/DISM scans or a Windows repair installation.