I would unplug all cables and replug firmly, particularly the big cable bundle to your mb, check for bent pins, check cable orientation as to pin #1 for all drives,reseat the cpu and memory as already mentioned above,clear the cmos. I might also remove and reinstall the mb to the case.
I just bought an Antec PP303 (300W) from Buy.com for $39 plus $7 ground UPS shipping. Got it in 2 days. It's not doing any better than the generic 250W that came with my case that's so generic it only says COMPUTER CASE in the box.
Good luck.