I've been struggling with this problem for a few months, my 1 year old laptop couldn't stand 100% CPU usage for longer than just a few minutes, or it would shut down because of the overheating protection.
It got much worse in the last few days, it actually shut down while being just idle on the Windows XP desktop! So I decided to operate.
And I think I found what caused it. The CPU has a weird heat sink system, there's some copper pipe connected to a basic heat sink, and on the other side there's some sort of chimney system, slightly reminescent of a bee's honey comb, and the fan could blow air through the pipes.
Now, what had happened was that dust had gathered up in the chimney pipes, so they no longer let much air blow through. After cleaning this dust out with a vacuum cleaner, and putting it all back together again, it seems to work well now. My CPU temperature indicator hangs around much more reasonable temperatures now than it has done in many months, but I haven't actually stress-tested it yet. I'm a bit wary of actually testing it out. But I'll have to, eventually.