What I do with infected computers in my shop is :
1.Pull the hard drive and attach it to a second virus free computer with a updated copy of norton av and scan and clean the drive
2. Check the report norton gives for the virus names that are infecting the drive
3. Replace the drive in the original computer and go to
www.symantec.com and either download removal tools for the virii I found or get removal instructions.
4. Follow the directions for removal - run Regedit and do the necessary registry editing or run the automated removal tool(s)
5. If necessary reload windows over top of itself to replace damaged system files
This procedure will get 95% of computers that are infected back up and running without losing data. Some viruses (like vbs.loveletter) are too destructive for this to fix - but most it will.