I had a computer that was not hooked up to the internet for about 4 months. I plugged it in to the cable and immediately went to update my virus definitions in Norton Antivirus. In the little window of time I was exposed I got infected with something. Norton dectected it but couldn't do anything about it - from what I was told the virus knew enough to disable it. I searched the web for virus tools and found several that offer a free virus scan from their site, as an inducement to buy their product. I used two of them (one I remember was penicillin) and deleted everything they identified. Then I used some registry cleaners (one of them was Ace Utilities, trial version) and I deleted whatever they recommended. Then I would go back to the internet virus scans, Norton's registry cleaner, some other programs such as AdAware and HijackThis, all the while booting over and over. Finally after two days I was free of the virus and its effects (it was a trojan and was designed to force frequent random reboots). But I did avoid having to reinstall operating system and software.