My wife's PC has had problems booting for the past year, but the last week it became unable to boot entirely. It started during a power surge and the PC turned itself off, despite being connected to a surge guard. Ever since then the PC boots seemingly at random, sometimes immediately, sometimes the fans spin for minutes/hours before it eventually boots or it doesn't start at all, the fans just keep spinning.
The problem doesn't really make sense since it sometimes boots without problems, so it can't be shorted or fried.
I'm not a real expert but have you tried to reset your bios? And have you tried the components in other computers ? Sounds to me that its either your psu or motherboard but then again I can't say for sure.