I must admit I have never heard of such a problem before, and I cant really think what may be causing it apart from a general build up of heat within the case. As you cay, you leave the PC on for "Long periods" so this may be the case.
Can I ask "How Hot"? Uncomfortable to the touch, warm, or just warmer than you would expect?
I too run an asus board with a 1.8 XP athlon and have no such problem.
What I would suggest is perhaps fitting a case fan or another one if you dont have one already. I have 2 in the front to draw air in and one in the rear pushing it out. My case air temp is 29.1 Deg C as I type this and the PC has been on since 04:30. (4 hours).
Try it, but unlsee you are getting some problems, which you dont specify, I would think this is the best option.