HELP*&!$!

I have a machine that every time an Admin of the machine logs in it immediately reboots. If you are NOT an admin it doesn't do this....go figure.....It was doing it about a week ago and I have since reloaded windows...it was ok for about a week, now doing it again...help.....

Machine specs: PIII-733 MHz.....10 gig HDD....128 MB ram......

Not sure if you need that or what other info might be helpful...any insight is MUCH appreciated!