I am having problems with random restarts.  Its just like I reach over and hit reset button.  I am running the latest drivers with all my hardware on XP Pro SP2.  If anyone has any ideas of anything I could be missing let me know.  System is not overheating.  My CPU and GPU are sitting at 26*C and 36*C at idle.  Memtest passed on ram with 0 errors.  System is not overclocked
 
Biostar TForce 520-a2+
AMD Windsor 5200x2 2.6ghz(no ocing done)
G Skill ddr2-800 2gb
MSI Radeon 3850 256mb
Antec Sp500 PSU
Xion Onyx case
Maxtor sata 3.0 160gb
 
Using on board realtec audio and realtec gigabit lan.

Could be the psu.
 
Write this on cd/floppy & boot it up. If the pc still restart in dos, something is screwy.
 
http://www.memtest.org/#downiso

My bet's on the PSU or RAM.

Ram test 100% ok with Memtest 2.0 from above.  I just noticed that I still had Windows set to auto reboot on system fault.  I just disabled that so next time it happens i may just get a BSOD.  If i do I will post a picture of it.