I had a trojan appear on the kids XP machine, (Backdoor.Genlot.DX) and went looking for info on it. Stumbled across this on the McAfee site and it appears to be a great free tool for helping kids surf safely.
www.siteadvisor.com
When googling etc, it puts up a big green tick beside reputable sites, and a big red cross beside those that try to do anything nefarious.
Tell your kids to only access the green tick sites and its a major step to securing your machine. It works with Windows Explorer and Firefox.