Is usually a sign of a failing video card. If this has onboard video try it without the 8800 video card.
i have an xps 630i with windows vista ultimate and it wont boot to the os. when i start the computer it shows the dell bios thing with dots all over the screen. then it shows a bunch of a's with accents on them(?). then it goes to the vista loading screen and after that it goes black with dots everywhere. this problem started when i was downloading vista service pack 2. my computer got stuck when it was at 53%.
intel core 2 duo e8500 3.16 ghz
corsair 4gb ddr2 ram
hitachi 500gb hard drive
windows vista ultimate 32 bit
nvidia geforce 8800 gt