Need Help With Persistent Crashes, Freezes, and BSOD

This problem began a little under two weeks ago. While playing Civilization 5, I began to notice visual artifacts appearing on my screen. I did not immediately interpret this as being a sign of an overheating video card, until my ATI video card restarted in the middle of a campaign. I proceeded to lower the graphical settings in Civilization 5, and the problem ceased. A few days later, I began to have issues with my desktop freezing up a few minutes after booting up (usually 10-15 minutes later). The problem usually ceased after one or two repeated boots, so I did not immediately take action on the problem. A week ago, I started-up Civilization 5, and 10 minutes after beginning to play it, my computer completely froze. After turning off my computer and booting-it up, my computer got a BSOD less than five minutes after start-up. Whether I was in the login screen or in an account, one of three things would always happen withing five minutes of booting my computer:

1. The monitor would turn off (indicating it was getting no input signal)
2. The computer would completely freeze
3. I would get a BSOD. The two messages I got when a BSOD occurred were either a Bad_Pool_Header or IRQL_NOT_LESS_OR_EQUAL.

Once these persistent problems appeared, I suspected action needed to be taken to diagnose if my video card was causing these problems. I managed to uninstall my ATI drivers (11.6), then install the beta drivers (11.7), to see if the problems may be related to my video card drivers. The problem still persisted. I then uninstalled the ATI drivers, removed my ATI card, and replaced it with my older Nvidia card. After the switch, my desktop was running smoothly. I even managed to play Civilization 5 without any problems. I was convinced those computer problems I had were directly caused by an overheated video card that was beginning to malfunction. However, two days after I switched my ATI card with an Nvidia card, I got a BSOD after booting my computer. The problems had returned. After doing some research online, I read that minidump files can be found after a BSOD, which can be useful to identify the source of such problems. I booted my desktop in safe mode, and before I could transfer the minidump files to my laptop, I got a BSOD, with a new error message (PFN_LIST_CORRUPT). Second time I entered safe mode, my computer froze and my monitor displayed a really messed-up image (mostly white with a red band on the top). The third time I successfully managed to transfer the minidump files to my laptop, of-which I found three (which are attached in a ZIP file to this post). Admittedly I do not possess the knowledge to thoroughly analyze these files, so I hope someone else can. I also read that using the Windows Memory Diagnostics Tool can check for hardware errors, and can be accessed through the Windows Installation Disc. So I booted from my Installation Disc, but before I could select the Windows Memory Diagnostics Tool, my computer froze (it froze on the "System Recovery Options" menu). After a second try, I was able to successfully select it, triggering a restart and the test to commence. After a few minutes, this message appeared:


“Status: Hardware problems were detected. To identify and repair these problems, you will need to contact the computer manufacturer.”


However, after reaching the 12% mark during Step 1 of the test, the progress completely halted. After 10 minutes of no changes, combined with not being able to access options or exit the test, I assumed that my computer had frozen, and powered off my desktop. I am near the end of my rope on ideas for identifying the cause of my computer issues. I really hope someone can help me fix these problems, and would greatly appreciate any assistance I receive.



Specs
Windows 7 Professional 64-bit
3 GB DDR2 RAM
1.5 TB Hard Drive (5200 RPM)
Initial Video Card: ATI HD 5770 1 GB
Replacement Video Card: Nvidia 9600 GT 512 MB