Random and Frequent Blue Screens

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: AMD Phenom(tm) II X6 1090T Processor, AMD64 Family 16 Model 10 Stepping 0
Processor Count: 6
RAM: 8191 Mb
Graphics Card: AMD Radeon HD 5800 Series, -2048 Mb
Hard Drives: C: Total - 953766 MB, Free - 125493 MB;
F: Total - 1430796 MB, Free - 275260 MB;
H: Total - 2861458 MB, Free - 2394523 MB;
Motherboard: ASUSTeK Computer INC., M4A88TD-M/USB3
Antivirus: Microsoft Security Essentials, Updated and Enabled

The problem started about 8 months ago, and I'm finally at wits-end with it. My custom built desktop has been fine for 3 years up until that point where it started getting blue screens. There does not seem to be a pattern as to what causes it, it BSOD on bootup, while idleing, and while playing games. It also occurs completely randomly, going days and even weeks without occurring, before starting back up again.

I have de-fragmented the three drives completely, and I feel like this helped a little but it might just be a placebo feeling. I have tested the health of all three HDD and they are all in healthy, working order. I have also run a memtest, but didn't notice anything out of the ordinary. I have also stress tested the graphics card with a FurMark to see if it would BSOD, but it did not after half an hour of running it on max settings.

Some of the Bug Check Strings I have gotten from the BSOD screens are:

- SYSTEM_SERVICE_EXCEPTION
- IRQL_NOT_LESS_THAN_OR_EQUAL
- ATTEMPTED_WRITE_TO_READONLY_MEMORY
- PAGE_FAULT_IN_NONPAGED_AREA

Thanks in advance!