Did you check Event Viewer? Always check system logs before trying anything else. I once had random blue screens, Event Viewer showed me the process related to the blue screen, turned out to be my antivirus (bitdefender). Check the log events with the times nearest to the system freeze, you might find the cause there.
no no no, i meant open the case lid and check for capacitors on the board, DO NOT open the power supply!
and safe mode is a good idea to start with, why didn't i think of that
try removing the ram and boot up on 2 gigs and keep overloading your system to see if it does freeze again,check the case,there could be a loose cable/wire.
Devil'sAdvocate wrotetry removing the ram and boot up on 2 gigs and keep overloading your system to see if it does freeze again,check the case,there could be a loose cable/wire.
He ran memtest86+, and it passed. Now you have a point in 1 thing: it could be the CPU's memory controller causing the issues.

@arithma: Run Prime95 blend, it tests a lot of RAM, and can crash or fail if there is a fault / wrong settings in RAM, memory controller, or CPU cores. Memory controller is stressed more with more than 1 stick of RAM.

Your S.M.A.R.T. results are excellent, by the way. But it goes like this: We know there is a problem if S.M.A.R.T. indicates there is (power on hours count / spin-up time / reallocated sectors), but even if S.M.A.R.T. indicates there are no problems, we don't know if there actually is or not. So in this case, it shows that your HDD is new, and *supposedly*, it shouldn't have any problems. Only possibility, to my knowledge, would be bad sectors, or loose wiring.
or maybe...its just his windows?
shant wroteor maybe...its just his windows?
Yes, but what are we able to do if it's Windows? He'd have to do a reformat, which is tedious compared to the other options to troubleshoot first. If the format didn't solve the issue, then he would have wasted a lot of time. In the meantime, while he's troubleshooting, he's also checking the system for any signs that can impact further down the road (heat, wiring, power delivery, BIOS settings, voltages, timings)
safe mode should be a good start,registry cleaner would be second and so on, i don't know about others but i usually start mine with a good old hard format (it seems to be a good solution for me as i eventually brake my hardware when i open the lid haha)
I have one peculiar thing that I found in CPUID Hardware Monitor: My +12V is out at 7.19V

@MrClass: All the critical errors in the Event Viewer are related to sudden power offs. My UPS has burnt down.
I'm currently running a Prime95 blend test. Will be searching for my driver CD or downloading them off the internet.

I'll revive this topic the next time my PC freezes after the format and while am on UPS.
Thanks everyone for the help, it was a very wild goose chase :) -- It's always nice to learn a few troubleshooting tricks and game.s
arithma wroteI have one peculiar thing that I found in CPUID Hardware Monitor: My +12V is out at 7.19V

@MrClass: All the critical errors in the Event Viewer are related to sudden power offs. My UPS has burnt down.
I'm currently running a Prime95 blend test. Will be searching for my driver CD or downloading them off the internet.

I'll revive this topic the next time my PC freezes after the format and while am on UPS.
Thanks everyone for the help, it was a very wild goose chase :) -- It's always nice to learn a few troubleshooting tricks and game.s
Software / BIOS usually does not read the voltage lines properly. Had your +12V line been below 11.4V (12V -5%), the PC wouldn't work, since PCs work at the ATX standard, which only allows +-5% deviation from the nominal voltages (3.3V, 5V, 12V).
Arithma i thought you had bsod did not know it was hanging.
From my experience (i had a cybercafe so i have alot of experience with weird problems) it is either a cpu or memory.
If it is psu then the computer would just switch off.
CPU try opening the case and check if the cpu is correctly installed and not moving.
Check if the cpu fan is not suddenly turning off and it is also installed well.
For memory you can experiment by removing a slot and testig first 4gigs then second 4gigs to determing which is corrupt.
I would think a freeze is more related to cpu then memory (memory usually bsod more) but with pcs these days who knows
ZeRaW wroteArithma i thought you had bsod did not know it was hanging.
From my experience (i had a cybercafe so i have alot of experience with weird problems) it is either a cpu or memory.
If it is psu then the computer would just switch off.
CPU try opening the case and check if the cpu is correctly installed and not moving.
Check if the cpu fan is not suddenly turning off and it is also installed well.
For memory you can experiment by removing a slot and testig first 4gigs then second 4gigs to determing which is corrupt.
I would think a freeze is more related to cpu then memory (memory usually bsod more) but with pcs these days who knows
If it is a PSU giving dirty power, or off voltages, then it is possible that the system could freeze. For example, a graphics card is receciving dirty power, it would crash, or get disconnected, the system would freeze.

CPU is passing Prime95 and not peaking over 70C, so I doubt it is a CPU problem. Even if not all the pins were connected, usually the ones at the edge would be loose, and I've seen a case online where (I think) 1 pin off was causing memory problems (4GB detected out of 6GB - BIOS level not OS level)

Memory has passed memtest86+, external to Windows, so if it was a memory problem, it could be a memory controller problem, and not something to do solely with RAM.

System seems to have passed Prime95 Blend test, which tests a lot of RAM and memory controller, so that seems to be fine too.

Actually, it's memory that is the prime, most frequent cause of simple freezes. CPU damage these days is quite low on the charts. Temps are fine, clocks are fine, no errors in Prime95.

However, for memory slots and moving around RAM sticks, it's worth a try, though even memory does not seem to be a problem.
Today after a long day of no freezes, I received the following BSOD:
A clock interrup was not received on a secondary processor within the allocated time interval.
I am googling it now.

The stop code was 0x00000101. There are a bunch of other memory addresses and other hex codes, but I guess they're not so important?
arithma wroteToday after a long day of no freezes, I received the following BSOD:
A clock interrup was not received on a secondary processor within the allocated time interval.
I am googling it now.

The stop code was 0x00000101. There are a bunch of other memory addresses and other hex codes, but I guess they're not so important?
Let's see what that is. It's new to me.
The thing I'd really like to get down to is to manually induce a freeze or BSOD, by stressing the PC in a legitimate way (not by replacing a driver with a corrupt file for example :) )
I need to get a UPS first, so I won't be formatting my driver yet. I had this issue though before a format, and after it. I've been struggling with it for a long time, but now I had the time to get down on it and try to solve it.
Can you please specify me your PSU please ?
@AvoK95: 
Cooler Master Elite 335 ATX  w/ Enlight 450W PSU Black      Operational Warranty     $80.00     $80.00
Get KNOPPIX or any other live OS and run it. If something goes wrong, you can be sure it's hardware, if not, then it's your MS Windows.