This problem started few weeks ago seemingly out of the blue. Basically, when I turn the PC on the harddrive won't start, in fact I can actually hear it trying to spin but never actually completely getting there, like a car engine that's not starting up. However, if I leave the PC just like that for a few minutes, i.e. at the bootup phase where it's trying to start windows but can't, and then turn it off and then on again it boots up perfectly fine.
At first I thought it was the harddrive but I connected another HD that I knew for a fact to be working fine and the same thing happened. So then I figured maybe it's the power supply, maybe it can't power the HD at first and needs to warm-up a bit before it can. The problem w/ that theory is that, (a) i didn't add any components so there's no reason why suddenly the PSU can't handle all of them; and (b) if I turn off my PC, after it's been working for a while, for 10 or more minutes, and then try to start it up again the same thing happens.
Is there a way to verify that it is or is not the power supply that's causing the problem? (I'm hoping it is because I have a feeling that if it's not the power supply it could be something much more expensive

)
Any ideas on what could be causing the problem other than the PSU?
My system is btw:
Athalon XP 3000+ (barton)
ASUS A7N8X-E Delux
1Gig RAM (512 + 2x256) Dual Channel, all Corsair
ATI Radion 9800Pro 128MB
Western Digital Caviar SE 200GB SATA-150