Is everything seated soundly in your computer (e.g. RAM, cards, etc...) Make sure cuz this can cause video problems. Do you get any Beeps on bootup?
Sounds like the problem could be your motherboard. :-(
Pentium 4 - 2 GHZ processor
MOTHERBOARD - Gigabyte Ultra GA-8PE667
VIDEO - NVIDIA Geforce FX 5500 OC
HARDDRIVE - 120 Gig Maxtor
MEMORY - 512 MB ram