My practical advice is to throw the hard drive into a healthy computer (that has antivirus software installed, configured and updated), run a no-holds-barred antivirus scan on the infected drive, then rescue whatever data files you can't live without, delete its partition(s) and shut down.
Now reinstall Windows on it. Afterwards, put it back in the system that's got custody of your data files, move them back to the drive, and there you go. Or burn them to CD or DVD if they'll fit. Try to get your defenses up better next time around. Firewall hardware, firewall software, and up-to-date, fully-configured antivirus software, plus all the Windows patches it needs, are a good start. Reducing or eliminating risky behaviors, of course, is also a good plan.