I just built a new system with these components:
Athenatech A449SL.400 Silver / Blue Steel ATX Mid Tower Computer Case 400W Power Supply
EVGA 122-CK-NF66-T1 LGA 775 NVIDIA nForce 650i Ultra ATX Intel Motherboard
Intel Core 2 Duo E6420 Conroe 2.13GHz LGA 775 Processor
MSI NX8600GTS-T2D256E-OC GeForce 8600GTS 256MB GDDR3 PCI Express x16 Over Clock Edition HDCP Video Card
SUPER TALENT 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 Dual Channel Kit Desktop Memory
While assembling it, I forgot to put the spacers in between the motherboard and case and the first time I powered it on, I smelled and saw smoke coming from a tiny chip on the motherboard. I quickly shut it off, realized my error, and put the spacers in where they belonged.
The computer was then able to start, but would frequently and randomly power off. It once stayed on long enough for me to install Windows. The random power offs continued though. Despite the motherboard problem I had, I suspected the cheap bundled power supply might be the culprit, so I went and purchased a Thermaltake 430W PSU. It seemed at first that this had fixed the problem - I was even able to play Half Life 2 for an hour or so. Then the power offs started again. The CPU temp never gets above 45 or so.
So having pretty much thrown out the PSU or heating issues as the problem, it seems like the only thing left is the suspicious motherboard that I had shorted. However, I would have expected a bad motherboard to prevent booting at all, rather than boot fine but experience random power offs. Anyone have an idea what might be going on here?