This is off topic, but whatever your reason are. UPDATE NOW! Running WinXP with SP1 on a machine thats online is like begging for trojans. Your system is most likely infected allready as every scriptkiddie that knows how to install a linux distro can easily "own" your system with metasploit. If you must have WinXP for specific tools that you can not get to run on a modern OS, use VM Ware.
As a person who also runs Win XP SP1 (for reasons unrelated to Bitcoin), I'm curious. Is there a real way to exploit such a machine (by a scriptkiddie or a real hacker) when the only Internet-visible port is tcp/8333 and the respective outgoing connections? Or is shorena spouting a standard pseudo-security paranoia? Please don't talk about social engineering attacks or other attacks involving physical access to the hardware.
Edit: Like Kelp! I also don't run antivirus on it, although I monitor it with a Tripwire-equivalent.