I'm running a P4 2.4Ghz.
My cpu temp pushes over 60C on hot sunny days in ATL. My SUV parks outside. Usually minimal shade and only factory tint. Console said outside temp was 96F. I'm not sure of the temperature inside the car, but definitely much hotter.
At extreme temperatures like above, my system sort of boots. It will begin loading windows but reboots right before showing the desktop. The system continues to reboot automatcally. If I roll down the windows and crank up the ac, within a few minutes I'll see the desktop. The cpu temp is around 58C.
I found a trick today though. When booting xp, I hit f8 and boot into safe mode. It loaded perfectly. Maybe it was a coincidence? I'll have to play with it some more.
Of course there's also middle ground. Sometimes I manage to park in the shade or its a cloudy day but still in the 80s Fahrenheit outside. Its not quite as hot inside. Usually the system boots on the 1st or 2nd try.
I have yet to find a solution. I'm not sure what the weak point is, but I'm leaning toward the hard drive which is rated upto 55C. I may throw in a few more case fans to see if it will help.