I have this customer who had this terribly infected PC. She is hooked directly to the internet through a cable modem, with original (pre-SP1) Windows XP, no antivirus or any other protection (like having sex with a homeless derelict in New Orleans without a condom) and it had every conceivable infection and was nearly impossible to clean out, especially since I had trouble booting into Safe Mode.
Finally, after almost 4 hours, I gave up and brought the thing home and work on it over a weekend. On Saturday, I couldn't boot into any mode. So I pulled the drive and hooked it into my own PC and tried to run Spinrite. Oops, Spinrite says the S.M.A.R.T. system on the drive says it is on its way out. I backed up what I could and checked the BIOS on the customer's machine, and sure enough...the S.M.A.R.T. monitoring in BIOS is off (my PC does not seem to have this capability).
So I called her and told her the bad news. I was able to get a new 160G drive (her old one was 120G with only about 20G on it) at Best Buy for $76 and now it is ready to give back to her, although all her programs will have to be reinstalled. She will have a software firewall now and AVG Free but can probably really use a router, since the bad guys have her IP address.