PC restarts after 15 min to 45 min......

I have a problem with my PC.It restarts after every 15 minutes or 45
minutes(Approximate time)..I have 98 and XP installed..It restarts in
both..I switched the system in DOS mode .There too it restarted after
15 minutes

Please help........

When it restarts, is it off for a while, or does it shut down and
restart right away? What kind of PC (motherboard, processore, etc)?

It's hard to guess/diagnose without more info, and hard to do even with
more details, but first I would check to make sure all fans are running
and the processor is not overheating.

In DOS mode? You mean in emulated DOS, or real DOS?
I could consider some virus in both OS's, especially when including
emulated DOS. Or a power supply frizting out, maybe the BIOS having
some glitch......

Make sure that your heatsink ismounted securely onto your processor and
make sure the fan is spinning. Check that you have enough thermal
grease and make sure there arn't any bent slots on the heatsink.

Boot up and go to BIOS. If you have processor temperature sensor,
observe it, many motherboards will turn off or restart system if the
temperature is too high. You may also have some background program
installed, or even one that you're aware of, which monitors temperature
(like MBM), and maybe it's configured to restart system when
temperature is too high. Also, some programs may have glitches, so they
may skip reading some sensor (it may be voltage sensor also) for just a
milisecond, and if they are configured to restart after voltage error
has been detected, that's it.
There are many posibilities, and you should check for everything that
has been written here (including viruses - consider installing
anti-virus software :: recomendation - Kaspersky).