Could be heat so try it with the cover off or as it's an elder PC we check for BAD CAPS (read http://www.news.com/2300-1041-5940552.html and similar), look at the power supply age and size and then try removing any hardware we can.
Since this is in the PC hardware forum I've dismissed any software cause.
I have a p4 gigabite mobo,2.00ghz,512mb ram, and the pc has been smooth untill last week when i started experiencing unexpected rebooting. i have changed the ram,powerpack, resetted the jumpers and the hdd is 80gig and is pretty new but still with no joy. The pc could run for a long time but just all of a sudden would reboot unexpectedly and sometimes within very short intervals. Am worried i dont know what else to do. pls i need ur help as usual. Thanks .