Originally posted by BlueOak
But I can't boot up the computer that might have a virus.
Yes. You need what is called a LiveCD. It is a disc image that can be burned to a CD or used to make a bootable USB drive. You will literally boot into the Live CD and run off of that instead if your hard drive. You will then scan your hard drive through that. I can give you more info on this if needed.
Other than that you may have a corrupted master boot record. The MBR is what the Windows uses to load the OS and configurations, and if it is corrupted then you may be screwed. You can fix it, if you are on windows, by booting from an XP installation disc and then choosing to repair XP (Also works for Vista and 7, but I am not sure of such an option of OS X or Linux OS's)
Beyond that, your entire system is potentially corrupted. I can't be sure.
Let me know more info. I am a student at a trade school working with computers and I could use the experience in helping.