*Help* XPS 700 "Hard Freeze" under 3d and even idle states

HI guys, ive done some extensive searching and talking to IT guys to try and debunk this frustrating issue, but if your reading this its because its not solved yet and need your help.

Im not a big gamer (although i enjoy alot of it) im a visual effects artist and not an enthusiast so i dont OC or go crazy with my system.

I have updated the XPS 700 since i bought it however:

• Western digital Raptor 10k rpm
• 8gb DDR2 ram (corsair)
• Standard nForce 4 SLI x16 that came with the 700 (not the 720)
• Upgraded geforce 250 gts 1gb ( i had the original 2x7600 gt's and changed them for a single geForce 9800gt with the same issue){the old cards 2x7600gt  where giving artifacts and distorting 3d }
• I also had the 1kw Power Suply that came with the 700

I have uninstalled all nvidia drivers in safe mode using driver sweper, and later installed the correct drivers. Cooling is not an issue ither, GPU-Z gives me 44c ~ 55c durring gaming or 3d aplications with extensive rendering. I have also ran to the advise of others "ortho" to test memory/CPU with no errors or warnings after a 28 hour test. I have not tested however 3Dmark06 as im afraid that it will just freeze like normal.

The Freezing issue is that while under a game, 3d aplication, or in some cases idle state, the computer will freeze. No ctl+alt+del, just basicly a screenshot no mouse movement ither, on top of that theres a high pitched note that comes from the speekers/headsets. I have also replaced the video cards three times to make shure its not a bad card : gone through two geForce 9800gt 1gb, and it still does the same with a new geForce 250 1gb.

Im at a loss here because im not OC (dont know how ) my hardware is all compatible and my drivers ar all up to date.....

Any help is so greatly appreciated.

V.