New graphics card, now hard drive isn't detected

Recently I decided to upgrade my graphics card from a 9500GT to a GTX 560 Ti. It arrived and I installed it into my pc, but a few minutes after booting up it froze and I had to turn it off. Same happened again and the next time I restarted I got stuck at the motherboard screen. I figured I might have damaged the hard drive somehow while I was installing the new card, so I ordered a new one and stuck in a 160GB 2.5" laptop drive that I had lying around until it arrived. All was well with the little 160GB drive.
When the new drive came and I installed it it just got stuck at the motherboard screen again. I tried sticking the old 9500GT back in with the first hard drive and everything was working again, the hard drive was obviously fine. I then suspected the PSU wasn't providing enough power for both the GTX 560 and the 3.5" hard drive to run together so I went out and bought an Antec PSU that provides 150W more power. Unfortunately this didn't help at all and I quit for the night.
Today I got up and everything was suddenly working. No idea why but I just went with it. A couple of restarts later after some windows updates it freezes while applying an update at startup and again I cant get past the motherboard screen.
I've really got no idea what's going wrong and I'd love to have it working so I can play Borderlands 2 tomorrow. Can anyone offer any suggestions as to what could be going wrong?
Thanks in advance.