PC switches off

Running:
XP pro
ECS K8 NVIDIA Socket 754 NFORCE3
AMD 2800+ chip with AMD fan
512MB 333MHZ crucial memory
Leadtek FX5200 graphics card
Seagate 80 gig SATA HDD
400 watt psu

Everything runs fine but PC switches itself completely off after being on
for hours but there is no pattern to it.

Doesn't appear to be a heat problem.

I suspect either a PSU problem or a motherboard setting?

Any ideas.

Just switching PSU to AMD 2500+ Barton on an ASUS nforce1 board with similar
spec to see if the same thing happens.

Noman

Wonder have you checked the error logs..?
Try clicking, Start/click Control Panel/click Perf. &
Maintenance/clicking Admin Tools to open/then click Event Viewer.

These are the log files which can be deleted or saved to a file for
printing or exporting, no doubt you'll be interested in Applications
and System logs, clicking these will open and may show you errors.

Could well be the PSU (Power Supply Unit) as you say if everything
appears to go dead, they are cheap enough to replace these days.

Maybe checking the bios will indicate something like an abnormal
temperature rise on the CPU, checking the fan might also prove
useful.

Davy