PoweringDownProblem

My Computer powers down on its own during use.

I am on Vista Home Premium with 2 gig RAM [+4GigReadyboost]
AMD 64 x 2 Dual Core Processor 4600+ 2.4GHz

Almost always the machine re-starts to opening page though occasionally shuts down.

Event Error Messages:

IRQARB: ACPI BIOS does not contain an IRQ for the device in PCI slot 13, function 0. Please contact your system vendor for technical assistance.
PCI slot could be 11,12,14.

Possibly no connection but also have had an on screen message:
NTDVM CPU Error

Hope someone can shed some light

TIA

Bill