Wierd Problem

I upgraded from XP to Windows 7 Ultimate x64 (complete uninstall/reinstall) when it first came out, and have been loving the way that it works with my components. However, after a hang on shutdown, my computer is not letting me boot into anything that involves Windows 7, EXECPT the built-in recovery thing (not the one from the disk). A more complete discription of what happens is that I turn on my computer and hear the POST beep, but when it gets to the Starting Windows screen it crashes my computer and I have to physically turn off or unplug the PSU to get my computer to turn back on.

At first I thought it was my PSU but I tested it and it can power my computer for hours in the BIOS, so the problem PROBABLY doesn't lie there. My current thoughts are that the problem exists with the Video Card (a Sapphire Single Slot 3870) or the power supplied to it. Having consulted with the eXtreme Power Supply Calculator, my PSU should give more then enough power to support my non-OCed system by over 100W, so if there is a problem with the power being supplied to that card, would using another 6-pin supplier work or would it be better to junk the whole thing and get a new one?

Specs:
Phenom x4 9950 @ 2.6GHz (Stock)
BIOSTAR TFORCE TA790GX A2+
4GB DDR2-800 (PC2-6400) 2*2GB Configuration
NETGEAR Wireless NIC
Antec SmartPower II 500W PSU