Quite possible that the PSU isn't giving you enough power. I assume the 75 GB drive you have is a Raptor.
Try taking out a stick of memory, disconnecting everything but your boot drive and your motherboard power plugs. Leaving your GPU connected, try booting and running some stress tests.
If you have an older video card, stick it in and see if it runs stable. 2.88v on the 3v rail is pretty low, but your PC won't draw much power from there.