strange problem

my computer is rebooting itself randomley, it does seem to happen more often when im online, or listing to mp3's. at first i figuerd it was a virus or maybe bad software or a driver problem or something. but now ive wiped out my hard drive then put winXP back on there and its still doing it. ive also tried this with a different hard drive and it does the same thing. so now im under the impression that it has nothing to do w/ the hard drive. this computer is not even 2years old yet!!! granted it is an emachines (dont know if they suck or not, but i remember it wasnt to expensive) any ideas of what it could be? or how i could maybe diagnos it more?? this is my first post here, any help is appreciated...thanx
-matt

 

sorry for 2 posts i should have typed up the specs.
# Microsoft® Windows® XP Home Edition
# AMD Athlon™ XP 2400+ Processor (2.00 GHz) with QuantiSpeed™ architecture
# VIA KM 266
# 256 MB DDR (PC 2100)
# 80 GB HDD 1
# 48x Max. CD-RW Drive, 16x Max. DVD Drive, 3.5" 1.44MB FDD
# S3 ProSavage8™ integrated (1 AGP slot available)
# 10/100Mbps built-in Ethernet
# 56K* ITU v.92 ready Fax/Modem
# AC '97 Audio
# Multimedia Keyboard, 2-Button Wheel Mouse, Amplified Stereo Speakers
# 6 USB 2.0 ports (2 on front), 1 Serial, 1 Parallel, 2 PS/2, Microphone-In & Head Phone jack on front, Audio-In & Out, 3 PCI slots (2 available)

 

thanx for the idea and link....ill post back when im done downloading an update of InCD4 that apparently could be causing this...(according to m$ error report) ill let u know what happens.