I have just built a new PC (yesterday) with Windows 7 OS, had a few teething problems installing Windows but eventually got there and everything appeared to be running fine, except a problem with no sound (pc not recognising any audio device) but I am presuming thats something to do with my sound card not being compatible with Windows 7?
This morning I have installed WoW, checked my mail and all seemed to be well, until when I returned to it, it had blue screened - first with:
"IRQL not less or equal"
then blue screen
"modification of system code or a critical data structure was detected"
The only things I am not sure are compatible are the sound card and the USB card I put in.
BSODs typically indicate a hardware failure, and in most cases, a problem with memory. The first error code typically indicates an overheated CPU or bad RAM. The first cause is the reason for the test for CPU temps. However, I'm betting it's some bad RAM because the second error code can occur if the kernal code is stored in memory that fails.
I would run Memtest86+ overnight and see what it returns.
Now that I have no idea on what's wrong. I would start with the memory test. I believe you have to download it and burn it to a DVD, and boot the computer from the drive, just like when you install Windows. If that doesn't give you an answer (or even if it does), I'd run the other testing programs in that building guide just to be sure everythings working right.
We need to know your complete system specs. Your RAM may not be faulty, you may just need to manually set the RAM speed/timings/voltage to their rated specs in the BIOS. There will be a sticker on the RAM with this information. There will also be a section in the motherboard owners manual on how to set the values. You should get the RAM settings right before running Memtest86+ since a lot of motherboards don't supply the RAM with enough voltage when using "Auto" settings.
Weird screen artifacts can indicate GPU problems - overheating, excessive overclocking, etc. But the other problems you've had don't really seem consistent with that. As shortstuff_mt noted, having your full system specs, as well as any info about overclocking, and what tests you've run so far, would really help people diagnose.