My computer freezes irregularly
Solution 1:
100 deg C is WAY too hot! It's possible that your processor has already incurred some damage. But in the interest of being optimistic, I'd say to run memtest86 for another 2 passes to be sure it's not the memory. Are you sure the timings and speeds are being detected correctly?
Did you check your motherboard for bad/puffy capacitors? If it's not your motherboard, then your PSU is either going bad on you, or is insufficient to power your hardware. That sounds like the most likely cause to me.
Use CoreTemp to measure CPU temps, since it's one of the most accurate programs around. Don't use ATI Tool, as it's incompatible with Windows 7. Try RivaTuner instead.
Solution 2:
First off, +1 for a perfectly documented question. This makes things AMAZINGLY easier for us to help.
You've done a lot of hardware testing to date and most come up with no problems. However, this may still be a CPU overheating problem (been there and it sucks). When you cleaned the dust, did you see any thermal paste between the CPU and the heatsink? If so, was it dried up or old? I recommend buying a small tube of Arctic Silver thermal paste (7$) and applying some to the CPU.
If that's not the issue, then I strongly suggest you start looking at your operating system and if it has any issues. You said you've run Fedora on it? I would recommend burning a Linux LiveCD and booting from it. Try using that as your OS for a bit, browsing and playing music/videos and stuff. If you don't get a crash there, that means that its either a Windows issue or a HDD issue (seeing as you aren't using it in a live environment). I would (from the LiveCD) run a disk check just to make sure. If everything comes up clean, we can safely say its Windows.
In that case, you need to determine if its your OS which is corrupted or you've installed something that is running some kind of service which locks your computer up. Give Windows safe mode a try and use that for a bit (I know, horrible resolution...I'm so sorry). If you don't get a freeze for a couple days then we can narrow it down to a Windows OS issue, in which case you need to look at backups or re-installation.
If you're re-installing your OS, make sure you back all your stuff up...
http://xkcd.com/612/
EDIT: While running in your LiveCD session, open up a terminal and type sensors. If that program is installed (Linux only), it will give you details about your power supply voltages, CPU temp, mobo temp, and everything else you need to know. Monitoring that while in your LiveCD session should give you strong indicators to whether this is hardware or software.
EDIT 2: Based on what you've said about running Fedora on another HDD via eSATA and not experiencing any crashes I would have to say that this is a software issue, maybe drivers. If you can run Fedora for, let's say a whole week without a crash, it's for sure an issue with Windows or deprecated/wrong drivers. How long did you run Fedora for? Did you try watching movies, playing games etc on it?
Solution 3:
The first thought I have is that your power supply may be going bad. Playing games or watching youtube may kick your grfx card into hi gear and increase power draw. Also watch those cpu temps 100 degrees ...yikes
Solution 4:
I have a laptop with windows 7 x64 ultimate that suffers of the same random crashes. I have noticed they happen mostly at home, when my G15 keyboard is plugged in. I do not remember to encounter those crashes when I use the laptop keyboard.
The G15 draws a lot of power on the USB plug. Maybe it has something to do with it ?
Solution 5:
Using the stability tester in nTune my computer freezed again (In the same manner as before). I notived that Speedfan gives me a -12V of -16.97V and a -5V of -8.78V.
Replace the power supply. These voltages are far out of spec and could be the cause of your problem.
Edit: The negative rails are rarely used today; however, the HWMonitor temperature is too high for temperature sensor 2 if the system is not under load. This could be a problem with the cooling system, motherboard, or power supply.