There's lot's of things that can cause a STOP 0x50 error (
http://aumha.org/a/stop.php#0x50 ). Have you tried the Startup Repair? What was the result (and any error codes that you may have received)?
Since you can't get into Safe Mode, I'd tend to think that the software conflicts aren't very likely here (although a Windows corruption could cause the same thing). With this in mind, I'd first suggest testing your memory with a bootable memory tester such as the free one here:
http://www.memtest86.com/ If that doesn't reveal a problem, I'd suggest a careful review of what was done to the system just prior to this problem (to see if it'll give any hints as to what caused it). Such as adding software or hardware - or changing or removing it. It could also have been caused by moving the system or hooking up new or existing hardware, power surges, etc, etc, etc.
After that, I'd try stripping the system down to the bare minimum to get into Windows to see if any of the hardware that you've removed caused it. Then it's a matter of testing the remaining hardware (usually by replacement of parts) to see if the problem is there. On a hunch, I'd test the video card first (since it's hardware and it has the Video RAM on it).