There is very little ability (less than 5%) even Kaspersky Virus Removal Tool or other this kind of tool on allive system can't remove all viruses.In your situation I suggest use special liveCD with antivirus for example this liveCD with Kasperky
Index of /devbuilds/RescueDisk/
This tool guarantee 100% to remove all viruses,boot this DVD and scan
But there is also possible you have already removed all viruses but they remains in your system blockades in Policies keys and of course any anti-virus tools don't remove these things - you have to do it manualy
Check you have following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate
if you find this key remove it + restart
If no help try to reset all restrictions in Vista:
How to reset security settings back to the defaults