It happens when I am in Windows, any program I use including anti-virus would stop responding in a minute or so. However, in boot time I could use Kaspersky to scan my computer, repair my computer etc and they wouldn't stop responding. I DOWNLOADED memtest to test my REM, and I managed to scan my computer within a minute timeframe both results were positive. Now I am curious of how to fix it as I still have some important files on the computer, I cannot WIPE my computer yet. Please help, much appreciated.

Here is a list of specs of my computer:
Friend built it for me, Windows 7 Professional 64 bit
Asus P8Z68-V PRO Motherboard
Intel Core i5 2500K 3.3GHz 6MB
G.SKILL 8GB RipjawX 12800CL9 2x4GB Kit
Asus 24x DVDRW LightScribe BK OEM
Asus Radeon EAH6850 DC 1GB GDDR5
Seagate 1TB 7200RPM SATA 6GB/s
OCZ Vertex 2 2.5" 80GB SATAIIHave you tried repairing your O/S?
MemTest needs to run at LEAST 1 hour...
What brand and wattage is the PSU ? ?press CTRL alt and delete to gain access to task manager, see how much of the CPU and RAM is being used.