Malwarebytes' Anti-Malware is getting good reviews, the free version only does manually activated scans and needs manually updating which is no big deal and you can pay for real-time malware protection and auto updates but if you also run AVG free version 8 that has a real-time resident shield which I find works well and it also has a link scanner that tells you if any of the sites returned by a google search have anything nasty waiting to attack you if you go there.
Be aware that AVG 8 needs windows SP3 installed on XP, it wouldn't install until I installed SP3.
http://download.cnet.com/Malwarebyte...-10804572.html
AVG Free anti virus.
http://free.avg.com/download-avg-ant...s-free-edition