I have no idea what's wrong with my PC....

I have a custom built PC. Athlon X2 3800+ , 1G RAM, GeForce 7300GT, Gigabyte AM2 motherboard, Windows XP SP2. Here is the problem:

Out of the blue, time frame ranges from a few minutes after turning my PC on to a few hours of it being on, my PC will all of a sudden become non-responsive. The mouse does not move and I can't click anything, the keyboard does not function. At this point, either 1 of 2 things happen:

1. The screen remains frozen until I manually turn the PC off.

2. I get a blue screen error. That most of the time says "Kernel Inpage Stack Error" "Beginning Dump of Physical Memory" With an error number 0x00000077. However, that error number is different, sometimes its 0x0000004A or 0x0000007A. But most of the time it is 0x077. At this point, my PC shuts down...

Then I try to turn my PC back on, and sometimes it says "Non system Disk error" When I try to start it up. So I have to keep restarting until it recognizes my hard drive. Then I start my PC up, and a few minutes to a few hours later it happens again.

Here are some of the things I've tried:

I reformatted my hard drive, re-installed Windows XP
Bought a new power supply (the old one was going out anyway)
Added two new fans to keep the system cool
I have 2 512MB RAM sticks. I used both individually in the system

After attempting all of these steps, I still get the exact same error. Any ideas what could be wrong?

Thanks for the help.