Okay, we have the same Mobo / processor (mine is delux wi-fi edition)
But I want to start there.
1) Have you changed the bios from default?
2) Have you updated the bios to 1002?
3) Have you reseated the RAM?
4) Did you overtighten the coolmem pipe on the Ram?? (I admit I did and ruined a stick)

I also have questions about the video card.
5) Have you reseated it?
6) Have you updated the drivers?

7) Are you able to test a different monitor on the system?

Instead of going into windows normally, have you tried Safe Mode??? (right after the BIOS screen (and beep), but JUST before the windows logo screen (or up to 1 second into it), press F8. Then select Safe Mode with or without networking

Safe Mode installs the low res drivers (just what's needed to run the system) for video, as well as minimal drivers for other stuff. It also performs some corrective actions.

I do not believe it's an overheat problem, but I've been wrong before.

Basically, what other tests have you performed?

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New PC :
New: Phenom 9500 Quad 2.2, 4gb ram XP Pro 64-bit (Sata 0), XP Pro 32 bit (sata 1), 40 eide HDD (music), Pioneer Blu-Ray Burner, HP Multidisk CD-RW/DVD-Rom, 2x250 Seagate HDD's, ATI Radeon 3870, Asus M3A32-MVP Deluxe WiFi-AP

Old PC:
P4 1.5GHz, 768 Ram, hda-80Gb (80Ubuntu), DVD R/RW