"Files required to use Windows Update are no longer registered or installed"
Fix that worked for me (XPSP3)
1. Download older Update Agent
2. Make a shortcut to the .exe file
3. Right Click the shorcut - select properties
4. On the Shortcut tab, add /wuforce after the path in the Target (example:"...\WindowsUpdateAgent20-x86.exe" /wuforce"
5. Run Update - It will act like "Microsoft Update" was never added.