W7 will not boot, crashes immediately - DESPERATE

PLEASE someone, help me! My PC is running the 7 Ultimate RC, build 7100. My problem is this:
When I boot my computer, it does the normal stuff, and then, about 5 seconds after my log-in screen shows, the computer just shuts off. There's no shut-down sequence, no warning, no BSOD. Just a straight *FSHEW*(that happy noise when something dies, btw), and it's gone. I press the button, turn it back on, same thing. It will crash like this anywhere after it first starts up. Sometimes, it only makes it to the second screen of boot info, sometimes I can log on, and it stays on for about two minutes, and then it dies. Anywhere between there, it WILL die. On a very, VERY rare occasion, it will stay on. I typically shut my PC down before I go to sleep, but after this began, I've been leaving it on sleep mode every night. That was after I wrestled for about three hours rebooting it over, and over, and over again, until it finally stayed on. It stayed on for about a month like this. Well, our power went out recently, so I had to go over the dreaded process again. It worked fine. I was shocked, and shut it down that night. The next day it was h-e-double hockey sticks trying to get it going again. It's been 'sleeping' like me every night since then. However, today is D-day for us RC users, and it expires in 18 hours, or so it tells me. Which means 2 hour periods of playtime. Well, I can't even get it to stay on for two MINUTES, let alone two hours. So this is going to be fun. I know, I should upgrade to the retail version. I'm trying, but cash is tight. I'm trying to find a way to get a discounted price on it, but there is no way to get that for at least another week or more. I've tried every fix I can, but I haven't found a repeatable fix yet. This all started around Christmas, I don't recall exactly, but I'm fairly certain it was after I upgraded the PC from a Fry's sale on the 26th. This computer was a gift from an IT man who understood my predicament, and gave me a basic, but good PC for a little physical labor in return. Well, the HDD was full, it was a little sluggish, and I had a little money to spend after Christmas. My PC's specs are as follows, with the post-Christmas upgrades noted:

Mobo: Biostar GF7025-M2
CPU: AMD Athlon 64 X2, 2400 MHz 4600+
RAM: Centon 2GB PC2-6400 800 MHz (NEW)
GPU: nVIDIA GeForce 9500 GT 512MB (NEW)
HDD: Western Digital 40 GB internal,(old) Seagate Free Agent 1 TB external (NEW)
Cam: Creative VF0415 webcam (NEW)

I suspect it's something with Windows, but I have no way of knowing. I tested different keyboards and mouses, but that got no better results. I also replaced the battery on the mobo, adn that didn't help. If you need more information, tell me, I'll get it on here. I really, really need help on this, it's driving me out of my skull, and thank you for your help in advance.

Well, I'm no debugger but:

I have a similar situation where my computer WILL run into fast BSODs (those that just flick and go away) after I shut it down and reboot the next morning. Currently I'm using a simple solution - after I boot and run into the first crash, I'll press the power button till the computer shuts down. Then I boot it again, in normal mode (it will ask whether to switch to safe mode) and it will never crash on the second go.

You could try that and post back. It's no real solution, but beats working a couple of hours on getting to log in.

Then again I have a somewhat similar problem with a non-rc version. Not as bad tho. But I've been too lazy to make a thread of my own.