There's an interesting interview on eWeek.com with Mike Danseglio, a program manager in Microsoft's security group, saying that PCs can get so bugged up with malware that sometimes you just have to wipe them clean and start over.
"When you are dealing with rootkits and some advanced spyware programs, the only solution is to rebuild from scratch," he said in the interview. "In some cases, there really is no way to recover without nuking the systems from orbit."
Some nefarious programs are so sophisticated now that they self-heal, he said, and actually will reinstall when you're trying to get rid of them.
"At Microsoft, we are fielding 2,000 attacks per hour," he said. "We are a constant target."