How do you check for faulty hardware?

I heared about people using Prime95 and OCCT, should if use both ? (CPU not overclocked just looking for problems)
How much time should i run these tests ? i understood that Memtest86+ is just an i/o test but not a strest test ? sorry if i'm off i'm pretty new to all of this.

Ok, i actually have 4 HDD's the ones i mentioned and another one of 1T, (S.M.A.R.T says their all good)
Should i disconnect it too ?
Also i read kind of scary stuff about stress testing and that leaving it overnight, can it be dangerous?

I see, any special settings i need on the prime 95?
I saw you recommended kombustor for the GPU test, but it does not check VRAM, what can i do for the VRAM ?
And memtest is only i/o test for bad sectors right ? what about a stress test or something, of is that all i need ?

Really ? where ? because as soon as it starts up i get the floating circle thingy and that's it . .
And what about the memtest ? is that suffiecient or is there another test needed to run ?

And if that prime number was a joke then lol and if not i'm confused

Wow ok, just another quick question i've got the 2 HDD's in raid 1 (mirror) i talked about and i configuered them in the bios but in windows he recognized them as a 1tr drive, that was untill i installed the intel Rapid Storage technology drivers, is it safe to reinstall the windows and disconnecting the raid drives ? or is there a special procedure that needs to be followed ?

Ok so prime95 worked for ten hours no problem, next i went to check the VRAM, it gave me a few errors but i don't really know what it means because there is not a lot of information on how to test the VRAM on your Graphics card, can anyone shed some light ?

Well before we get to the VRAM i did the a few tests -
Memtest - Ran for 4.5 hours with three passes and no problem with my RAM.
Prime95 - Ran for 10 hours overnight with 50-55 temps over night (assuming the sensor is correct) and passed, all good.
Then a friend of mine said occt is a much more efficient tool at checking the CPU, i ran it in the CPU:OCCT option and it failed after 24min giving me an error with no specifics (no heat or core details), then after a few hours i ran it again and it failed after a few seconds with an error on core 1 . . . then i switched to the OCCT: LINPACK and it ran for 30min before crashing i marked the three options (64bits AVX Capable Linkpack and Use all logical cores, as been suggested to me by a friend i don't really understand what option 2 and 3 mean).
Should i be worried?

The game took a back sit to be honest, since the last time i posted here's what happened -
I wanted to get to the bottom of all the issues so i striped my PC bare and reapplied thermal paste on the CPU and ran it only using one stick of memory, the video card and the SSD, i disconnected everything else and re installed windows with only drivers installed.
Then i ran OCCT again in both CPU and Linpack modes and passed both times with an hourly test.
Then i added another stick and yet again i passed the test and everything was looking good, then i decided to add the other 2 sticks and test it again . . and the PC failed to boot, after a few hours of troubleshooting i have one memory slot that just magically stopped working . . i mean i tried different memories also cleaned it with a electronic contacts spray and it just refuses to use the slot, now as i said just a few days ago i passed a 10 hours test on Memtest with all 4 sticks attached, and in the process of trouble shooting he finally booted up with the 3 memory sticks but only in a particular order if i didn't use that specific order it would not boot up which is weird . . Now i have the asus sabertooth P67 MOBO and my bios is updated to latest version (everything worked with this bios version before) i tried the MEMok button a few times to no avail and tried different memory sticks and cleaning the slot the one thing i didn't try is clearing CMOS since as i mentioned in the beginning of the post i have a RAID 1 configuration that was configured on the bios and i have alot of important information there that i'm afraid to lose if i wipe out the settings, but it seems unlikely that it would solve a problem with a single slot . . but anyways i gave up on it and remained with my 3x 4gb corsair vengance 1600mhz instead of x4.

Then comes the funny part . . i ran the OCCT test again and failed ones after 10 minutes with error on core 0 and then after a few seconds with just an error and then yet again after 5 min with error on core 3 . . i was surprised to say the least . . started digging around and foundout that my board has a turbo mode that overclocks my CPU from 3.4 to 4.1, i have the i7-2600 3.4ghz sandybridge CPU and i was under the impression that it could not be OC'd only the 2600K can be OC'd so with little knowlage of this feature on my board i tried to reset the settings and yet again i fail the OCCT test . . then i thought ok maybe i damaged one of the sticks during all the testing so left it over night again with MEMtest and yet again i passed with the three sticks for over 10 hours.

Sorry for the wall of text but i really don't know what to do, every time i try to solve one problem a new one pops up, and i didn't even get to the GPU which the test for the VRAM also throws errors too
Maybe my MOBO's turbo featured made a conflicting adjustement . . i think i put it all back to default though . .
Any ideas would be much appreciated !

Ok update:
It seems i pass the OCCT test with a specific ram sticks while with one of them he gives me an error, which is wierd because all sticks have past the memtest, so since i have one slot that is busted i just won't use that stick for now.
Does anyone have any idea what could have happened to the slot ?

btw can i run HD tune pro for a raid 1 configuration ?