|There are some nasties out there that are known to diasble/disallow access to any installed virus utilities etc. Thus I doubt you can/will easily be able to install another and scan etc. Also if the original utility is still installed and set as active... likely the newer one won't go in and work properly (even if the current one is apparantly ineffective); or you will get a range of other possible problems - all related to having tow a-v utils installed and "active" after a fashion... Some of these nasties have been found to really have a go at the registry tooo; which is often why you can't access yoiur install a-v utils and maybe other similar pest controllers...|
Boot with a linux variant on CD/DVD - Knoppix or Ubuntu are the usual favourtes here...
If you don't have either.. go to Knoppix.com or Ubuntu.com; download the .ISO (image) file. Save to a hard-drive, then burn image to a CD/DVD; and use that disk.
Once up and running... go on-line to trend.com and run their freebie "housecall" utility. Opt for " full" scan. Once you're satisfied it's running... go make a tea, coffee etc... and wait for results.
If it finds anything let it fix it; likewise if it per chance quarantines "anything" - locate and delete it.
Then reboot system as per-norm (remove linux disk first) and see if things are now OK?
Presuming you are able to resolve it as above... afterwards update your current virus defintions and rescan using that utility. However... before runnng a scan using a locally installed utility... disable system restore first. Even better scan in safe-mode again with system-restore disabled.
If utility finds "anything" and likely may quarantine it too... locate and delete the quarantined files etc...; afterwhich empty Recycle-bin. Then reboot and see if all is OK. If it is re-enable system-restore.
One disables system restore so as to prevent any nasties that may be deleted/removed etc. beig re-inserted/re-installed by system-restore...; and likewise cleaning out quarantined items/nastties prevents them from sneaking back in...