I’ve also found A squared good for killing trojans/spyare: http://www.emsisoft.com/en/software/free/ you might want to consider a resident antispyware shield, such as Windows Defender. I use SpywareTerminator and find it very good.
Ditto the thoughts on the firewall. One that blocks outbound is a good idea, the inbuilt XP blocks only inbound AFAIK).