Online virus scan.

Some viruses are so destructive that they can shut down antivirus program preventing you from scanning your PC. Even if you try to re-install the antivirus program, you are restricted to do so. When you encounter such kind of situation, there are in fact a couple of ways to scan for virus. You can detach your hard-drive, get it mounted on another virus-free PC with antivirus program installed and scan your drive. Or maybe it will be helpful if you repair your Windows and re-install antivirus program. But I suggest you try to scan for virus online if you can connect to internet. Maybe that option is your last resort. Here are good sites for online virus scan:

http://housecall.trendmicro.com
http://www.kaspersky.com/virusscanner
http://www.bitdefender.fr