I have a HTPC that I just upgraded to windows vista. Literally as I was installing my drivers on the machine 60 minutes after the OS install, the computer shut off and I have not been able to turn it back on.
I checked the cords and wall socket and they are fine. when the computer is hooked up, the standby and power-on led on the motherboard turn on but the fans do not start up (either on the cpu/system nor the power supply). I checked the power supply unit by using a wire to cross the 24 pin socket and hooking it up to the dvd-rom drive and it powered up fine.
Here is the kicker. I took the motherboard out of the case and stripped it down to just the cpu and reseeded the RAM. I then hooked up ONLY the 24 pin atx power connector and, viola, the cpu fan, psu fan, and motherboard fan are functioning. If I power-down and then reconnect the 4 pin 12 volt power connector, then the same thing happens (leds on but no startup/fans). The same thing happens if the motherboard is screwed back in the case (fans start with 24pin atx power only but don't with 4 pin 12 volt connector added).
I really don't know if this has anything to do with the software issue. Usually I am pretty savvy about diagnosing these problems, but right now I am stuck. I am wondering if it is a motherboard issue or a problem with the voltage regulator module as that is part of the +12volt system.

Here are my components:
DFI Infinity N4 AMD socket 939 motherboard
FSP AX400-PN 350-400watt power supply
AMD 3500+ 64 processor
4x512mb dimms
300gb sata 1 hd
lite on dvd burner
Thanks in advance for the advice/feedback