Fwiw, my XP system Never gets plugged into the network. I only use it for audio work and photoshop. I just don't want the hassle and time to fix it when (not if) it gets infected. As a result of this conservative strategy, it Just Works. I use linux for everything else.
I even have an old system (linux) that I dedicate for p2p. That's all it does, p2p. A lot of p2p apps are resource intensive when they are very busy. I don't do as much p2p these days so performance concerns are less of an issue. I use azureus and disable all of the auto-update functions, etc. There may be better choices under xp.
I hope you run a good hardware firewall and also a software firewall. There are vast numbers of websites that can infect your browser. Also, a lot of 'reputable' websites get infected and can infect your system. Of course apps can fairly easily bypass software firewalls. The banner ads on 'trusted' websites often come from untrusted sources. Those banner ads often contain javascript and flash, bad news all around.
If I ran xp as my primary desktop I would probably only do so within a vmware environment... but I'm paranoid about these things
