Once again...it is your Celeron.
But check to make sure your AA and ANISO are turned off. On the desktop right-click to bring up the properties. Settings - Advanced on the AI screen click on 3D and move the two slide bars all the way to the left. Try that, if you haven't already.
Celeron's aren't gaming chips and especially on DOOM 3. It might be enough to play Counter-Strike, but not the better engine games like DOOM 3, FarCRY, Splinter Cell - Pandora Tomorrow, Half-Life 2, etc.
The rest of system should be good enough to run DOOM 3 at low to medium quality at 800 x 600, if you replace the CPU.
Soyo Dragon Ultra Platinum Edition powered by a P4 3.06GHz w/HT @ 533FSB
ATI Radeon X800Pro 256DDR <---Yeah!!! I finally got it!!!
1024MB PC3200, Creative Audigy LS, 2 x 40GB = 80GB RAID 0
Windows XP Pro, DOOM 3, CS, UT2004, BF1942 RtoR, CNC Generals
Finished Half-Life 2!!!!!!!!!!