Many people I've talked to lately have either had some kind of spyware infestation, the notoriously scamacious Antivirus 2009 (or 2008), or just something that the kids did weird that messed up their computer.
In looking around for something to help keep computers under control, I found Windows SteadyState, which is a free program put out by Microsoft for locking down Microsoft Windows. As much as I dislike some of Microsoft's business practices and their frequent security problems (stop laughing Mac fans - the Mac OS has been bitten by malware as much as Windows has lately), this is a program that appears to have what it takes to really lock down a computer. It won't help in cleaning one up after the fact but in keeping it from getting messed up in the first place.
I used it to create an account for the kids. The account is locked down so no programs can run except Internet Explorer. Then IE is locked down so it is more limited than normal. You can set it up with a whitelist or only specific sites that can be visited, but I didn't turn that feature on.
If something strange does get installed even with the limited version of IE that is running, when you log out of the account, all changes made to the computer are automatically removed. Pretty cool. You can unlock the account so you can make changes and then just lock it back up.
I'm still playing with it, so I don't have a full review for it yet, but I recommend trying it out.
If you don't want to totally lock down your computer to just a small list of websites but still want good protection, I recommend K9 Web Protection from Blue Coat Systems. It's free for personal use. It lets you pick from a huge list of categories of sites that you can block and logs all sites that are visited.
And if you do happen to get the Antivirus 2009 trojan installed on your computer, I've found System Restore, which is automatically enabled in both Windows XP and Vista, to be the easiest option to remove it.