XP SP2 machine suddently turning itself OFF

[KWW]

Hopefully this is not a HW problem... Win XP SP2 Pro, all the latest
patches... recently has occasionally started to just turn OFF or reboot for
no apparent reason. Ran full virus scan (NAV) and Spyware Scan (MS AS beta)
and nothing turned up. The other day, suddenly, it said "network connection
lost"... but the cables were still connected and my other networked
computers were connected.

Win XP Pro, SP2
Soyo Dragon + (SY-K7V)
Built-in sound and LAN
512MB RAM (DDR 2700) Kingsford
120 Gb Hard Drive (WD or Maxtor, not sure... not on the PC at the moment)
GeForce4 MX 400 64MB or 128Mb (can't recall), AGP
APC Battery BU P/S (not connected via network cable)
Internet via Cat 5 to router to Broadband connection.

Any thoughts would be appreciated.

Thanks.

 

[NotMe]

Right click My Computer>Properties>Advanced Tab>Startup & Racovery
uncheck the automatically restart box.
then post the error you get.