OS Lockup (on fresh install)

99% certain that a piece of my hardware is causing major issues here, but I'm stumped as to what, and why.

I built this spare PC from parts from my past rigs... It freezes at random points, sometimes just while idling, sometimes while working. I can move the mouse around but there is no response from clicking on anything or pressing any keys. only solution to power it down is hold power button or switch off PSU.

SPECS:

Pentium D E6700 3.2GHz

ASUS P5QL-PRO on newest BIOS revision

4GB DDR2 RAM (2GB Gskill 1000Mhz downclocked to 800 to match the other 2GB Patriot 800Mhz stick)

Seasonic 450W PSU

Seagate momentus 7200RPM 500GB HDD

EVGA NVIDIA 8800GT Superclocked edition

Win 8.1 Pro 64-bit

-HDD Tune offers no health warnings on the drive, yet just before the rig freezes, Resource Manager shows disk I/O usage at 100% yet nothing is using over 0.3mb/s.

-Matching sticks of RAM from two different kits is awful, im aware, yet this problem still occurs if i use either the patriot stick or the gskill stick..

-PSU fan is seemingly maxxed, could it be a power issue? not much sag on the 12V according to HWMonitor...

Brainstorm with me! I love you guys <3

via Bleeping Computer Last 20 Posts http://www.bleepingcomputer.com/forums/t/512899/os-lockup-on-fresh-install/