In the event that you are not able to install updates usign WindowsUpdate you can follow any of the following procedures :

Use a utility like ccleaner clear up PC Junk. Also flush the Software Distribution Folder and then reboot. If you are using WindowsUpdate try using MicrosoftUpdates and vice versa.
If you still cannot Update Windows Vista (or WinXP/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 :
http://*.windowsupdate.microsoft.com
http://*.windowsupdate.com
http://update.microsoft.com
http://update.microsoft.com/microsoftupdate/v6/default.aspx
Click Close > Apply > OK. This is known to help the GenuineValidated Windows, in some cases, but not always.
Try to re-register the following dll’s individually. Open Notepad. Copy-Paste the following command lines into it and save the file using a .bat or .cmd extension.
regsvr32 /s wuapi.dll
regsvr32 /s wuaueng.dll
regsvr32 /s wups.dll
regsvr32 /s wups2.dll
regsvr32 /s wuwebv.dll
regsvr32 /s wucltux.dll
regsvr32 /s wudriver.dll
Double-click the icon of the .bat or .cmd file; command window will open. Run the commands, and then close. Reboot, and see if Windows Updates is working OK. Also Open IE > Tools > Internet Options > Advanced > Security Settings > Ensure that the two boxes “Enable SSL 2″ or “Enable SSL 3″ are checked. Hopefully, this should help !
Else, note your error code and visit this excellent Microsoft Resource : Windows Update Support: Error Codes. Search for the solution against your error Code. If you receive Windows Update error 80246008 while downloading updates, you might need to change the Background Intelligent Transfer Service (BITS) or Windows Event Log service settings, and then restart each service. Click here for more on it. KB943144 is another helpful link.
If you are recovering from a malware attack, you may also want to have a look at your Hosts file. Some malware programs may modify the hosts file to control DNS name resolution. Remove ALL entries for Windows Update and Microsoft Update from your Hosts file situated at C\Windows\system32\drivers\etc\hosts. You can either remove any lines that contain entries for Windows Update or for Microsoft Update OR you can just place the number sign # before or in front of any line that contains “microsoft” and “update”. HostsMan is a good freeware utility to help you manage your hosts file.