Windows Vista computer turns on and off, not even going to boot screen?

The computer will turn on, 2 of the fans will work, but it shuts down within 15-20 seconds without going to a boot menu or anything. It doesn't sound like the power supply fan is turning on, just the two other fans. I had this problem before and I'm pretty sure HP said it was a motherboard problem, but I can't be certain. Any advice or questions to help someone answer better would be appreciated. Thanks in advance.