try to narrow down the process that stops the virus from loading (isolate). (usually an EXE file)
then google that process for a solution.
Or enable the virus, reboot and try one of the many free online scanners available to see if they can pick it up. If they can't solve it, you atleast know the name of the virus and can google for solutions.
http://www.bitdefender.com/scanner/online/free.html
http://www.eset.com/onlinescan/
http://security.symantec.com/sscv6/WelcomePage.asp
http://www.windowsecurity.com/trojanscan/
and if all else fails, then reformat.