PC restarts almost randomly?

Hello, and thank you for taking time to read this question... Ive been experiencing what i would call the most annoying thing a pc could possibly do. It restarts without warning, and almost randomly. By almost i mean there are (some) consistencies, for example- it seems to happen when im scanning for... show more Hello, and thank you for taking time to read this question...

Ive been experiencing what i would call the most annoying thing a pc could possibly do. It restarts without warning, and almost randomly. By almost i mean there are (some) consistencies, for example- it seems to happen when im scanning for viruses(after 2-4 tries it usually gets all the way through the scan) and when i listen to music on WMP, in any situation ill usually last about 10-20 minutes (20 being a good day) in WMP before it goes into power saving mode, and immediately wakes back up to re-load windows. It also happens in firefox, although less frequently. But sometimes it will be asleep and for no apparent reason goes into power saving mode, and re-loads windows. No information is ever lost, files, folders etc. and i've scanned the entire HD repeatedly and found 0 threats.
i can contribute the problem with either my power supply, lack of RAM, bad processor or over heating or a combination of the given.
pc specs-
ati radeon 2400 pro HD (PCI bus type)
year 2004 Intel pentium 4 processor
year 2004 Intel motherboard
256 mb RAM
250w PSU

please help :)
Update: ive also reformatted the PC a few times for other viral reasons, and it's still doing it, which in my opinion rules out any software issues (registry files, OS components etc.)
Update 2: P.S. it's a desktop NOT A LAPTOP
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