I was having a serious problem with the rebooting not only in SHIII, but I was able to get to the bottom of it in the end, it turned out to be something incredibly different then I had suspected.
Anyway, what you have to do to figure out what is wrong:
Go into settings -> control panel -> administrative tools -> event logger
Click on the system on the left and you'll see a history of all the errors on your machine. Scroll down until you see an "informaiton" with the added part "minidump" on it. Double click on it to see the details. There will be a number followed by a parentheses (it's in hex) the number you want is the one preceeding the parenthesses
Take this number and go to the website
http://aumha.org/win5/kbestop.htm
There will be a listing of stop codes. Find the code and read what it says. It turned out for me that my RAM had several errors on it that I only detected through a memtest program
http://memtest86.com/
I removed the defective RAM and now I haven't had a problem since, but I have to go get some RAM :P Good luck with your difficulties. A lot of what I read on the subject on the windows forums is also related to heat in a computer as well as old drivers conflicting with new ones.