Broadly speaking, you can try one or all of these :
Run CCleaner to clear up PC Junk. Reboot. If you are using windowsupdate try using microsoftupdates and vice versa.
If you still cannot Update Windows Vista (orWinXP/SP2) using windowsupdate, try this :
Open Internet Explorer > Tools >Internet Options > Security > Click ‘trusted sites’ icon >Click Sites > uncheck ‘require server verification’
Then add following sites :
Click Close > Apply > OK. This is known to help the GenuineValidated Windows, in some cases, but not always.
Else, lastly, note your error code and visit this excellent Microsoft Resource : Windows Update Support: Error Codes. Search for the solution against your error Code.