Part of the problem is likely that there are just too many known vulnerabilities in older OS’s.. It may help to make sure you’re fully patched up to the last patches for XP. But the end is the end... And there won’t be more. There will always be risks for such machines, and they’ll need to really strictly firewalled and locked down.
The crappy “Windows Virus Protector” imitation virus sucks ass. I got it after a mistype of facebook and it popped up. Ctrl+Alt+Del’ed firefox but the sucker already installed through javascript or something.
It was a nasty rootkit virus, too, but now it’s dead. It’s just amazing the crap they make nowadays.