I hate to sound like an asshole, but have you ever used a computer before?
Ok, the MINIMUM specs for Doom III are
-graphics card with at least 64 MB RAM that supports directx 9.0
-Pentium 4 @ 1.5 GHz or Athlon XP 1500+
-384 Mb RAM
-8x CD ROM drive, and 2.2 gigs of uncompressed space for the game files
Right click on My Computer and select 'Properties'
Your basic specs will be listed right there at the bottom. i.e. - processor speed, megs of ram
If you're running windows XP, click on the 'Hardware' tab, then click on 'Device Manager'.
Now click on 'Display Adapters' and you'll see twat kind of graphics card you have.
NOW, if you are sure that you have a computer fast enough, right click on the graphics card listed under 'display adapters' and select the 'Drivers' tab. Either update the drivers that way, or manually go to the manufacturers website and download them from their webserver.
Either THIS is the problem, or you have the graphics settings set too high(which also ties in with what I just wrote above about not having a fast enough rig). If I am not mistaken, there should be a "detect best configuration based on system specs" or something along those lines under "options" and then "system". Check that out and do it. Also, if you don't have a great graphics card, turn off anti-aliasing. It's nice, but don't worry about it unless you are hardcore.
Wow, I feel like a total computer nerd. Mainly because I am, but oh well.
I hope this helps you out, CZeplin, and if not, feel free to ask more questions and if I can't help, maybe someone else on here can. Laterz
--There is a wrong way to eat a Reese's, and I will find it.