Vundo and fake anti-malware software
By far the number one consumer/home user problem I see is fake antispyware/antivirus software that holds a system hostage.
At the core of this problem is the VUNDO trojan. See http://en.wikipedia.org/wiki/Vundo_trojan
In my tests this Medusa’s head malware infestation is best remedied by zero filling the hard drive and installing from a trusted source.
In past tests the popular countermeasures Ad-Aware and Spybot were useless as was Norton’s Vundo removal tool.
Today we tested Malwarebytes Anti-Malware. After five passes the product declared the drive clean and indeed the symptoms were gone. I removed the drive and mounted it in a test machine and ran Avira Antivir against it and found three more instances of Vundo.
To me this confirms that you cannot be confident that malware has been removed even if your tester shows a drive clean and confirms that the best approach is wipe and reload.