pc won't boot from cold without reset

Just built a modest PC - and it functions 100% fine on all games aplications etc - but if I leave it off over night, when I press the power you can hear it switch on, the fans come on, but no beep and no monitor output - it doesn't boot until I hit reset. And then it comes on fine in almost exactly 1 minute.

It switches off with the power switch.

If you leave it off for an hour or 2 it'll come straight back with the power switch.

My computer friends are stumped and so am I ? Mother board error? Case error?

If anyone has any idea I would be very grateful.

Thanks.


LG-6690 ATX Casecom Piano Black Mid Tower Case with Red 120mm Fan
ABIT A-S78H 780G Socket AM2+ onboard VGA 5.1 channel audio mATX Motherboard
OCZ 4GB (2x2GB) DDR2 1000Mhz/PC2-8000 PLATINUM Memory Unbuffered CL5 DUAL CHANNEL
Gecube HD 4850 512MB GDDR3 Dual DVI HDCP HDTV Out PCI-E Graphics Card
Arctic Power 500W PSU With PCI-E 2x SATA, 20+4 ATX12V 8pin +12V Connectors - Retail Boxed
2 - 250gb Samsung sata hard drives.
DVD Writer
DVD Drive
Vista Home Premium SP1

Had a Epox MB that would do this. It turned out the problem was in the BIOS. It had a "Health Check", it checked the CPU temp's. When it reached this point in the Boot process it would lockup and require a reset. After shutting off this setting , never had the problem again. Dont know if this is your problem, but I would see if there is anything in the BIOS that is hanging it.

For me a simple bios update helped (on an Asus P5n-32 Sli), my MSI P6n Diamond never had this problem.