Personally, I would take out the hard drive and mount it in an external case, or as a slave drive in another system.
Then, run a full scan of the drive with both anti-virus and anti-spyware programs.
Then, while I had the drive loose, I would make back up copies of the users files as best you can.
Make sure you get the mail and address book files.
After cleaning and backing up the drive, I would mount it back in the system and see if it now boots.
If not, wipe the drive and start over.
Sometimes, it is just not worth the time to try and clean up a system.
Make sure to install an anti-spyware app as part of the rebuild.
Chas