Geoforce 8600 crashes the whole PC (both Win and Linux)

Hi all, I have a desktop PC with an AMD dual core processor, ASUS M2N4-
SLI motherboard, Nvidia Geoforce 8600 GT and an HP 22207h 22 inch
monitor. I run both Ubuntu 64 bit and Windows XP 32bit. Lately I have
been experiencing quite a few problems with my video, both in Windows
and Linux, whereby I start seeing little green or red dots all over
the screen, and after a few seconds then the whole PC freezes. If I
reboot, I still see them when the bios boots up; the only way to get
rid of them is to switch everything off, wait a few minutes and then
switch back on.
Any ideas what it might be? I updated all the drivers, but the fact
that the very same problem happens in Windows and Linux, and doesn’t
disappear after a reboot (i.e. I still see these dots even before any
operating system has booted up) makes me suspect it may be a hardware
fault with the video card or the motherboard.
Thanks!