Free software you can use to defend Windows XP
The main things are to make sure you have installed the XP SP2 service pack and Internet Explorer 7, which provide a dramatic increase in security, and that you are running a firewall. (You should install IE7 even if you use Firefox.) It’s also a good idea to have auto-updates turned on. You can survive with these, if you are careful: I’ve done it. But if you can be accident-prone or surf some of the less savoury parts of the net, it’s important to install an anti-virus and an anti-spyware program.