I always try to keep the fan clear of dust or other things that clogg it up. Because it does heat up...
Mine heats up on the left front side (where the palm of my hand is) when it's in sleep mode (when he's closed).
Maybe you could try to put them both on a partition, and try to monitor where/when/how it heats up on both OS. Then you could locate the problem to a specific place, and maybe the hardware near it. This will make it easier to figure out where (or wich driver) it goes wrong on XP...
I'm no pc expert, but I'm trying to learn here and there, hope this could help you a little bit...