Assuming that you have access to some suitable hardware, I would try to
replace some hardware items one at a time. After trying each item put the
original back and try the next in this order
1. Power supply (Most likely) you could also reduce the load on the PS by
unplugging CD drives floppies etc just keep the bare essentials i.e Hdd and
video
2. try a different Harddrive in the suspect machine. You could also try that
Hdd in another machine but be aware if you go that way you my stuff up the
installed drivers
3 disconnect one fan at a time and see if system goes down quicker, although
if there is overheating you should get some warning beeps. Make sure you
have a speaker plugged in to the motherboard most old towers/mbs had one

good luck

"Gre0145" wrote in message
...
A friend has an old PC running 98SE. It will power up but after 1/2 an
hour to 1 hour it will die and not turn on. If you let it sit for
another 1/2 hour it will power up and repeat the process. Power Supply
or CPU/Motherboard? All fans are working and I have blown all the dust
out of the tower. Thoughts?