I'm still confused by this Dell system having a Quad Core and GeForce GTX 550 Ti coming preloaded with Windows XP. Was your system a custom order from Dell, where you were able to ask that they install XP as the OS?
I see that the board in that is based on the nForce technology. Are you using its nTune software to overclock anything in any way?
To answer my earlier question, it appears as though your PSU is an OEM Dell 750W. I'm assuming that it's about four years old?
Have you updated the Dell BIOS at all on this system?