Doom 3 wont start new game

Hea,

I jus bought Doom 3 and installed it, but I seem to be having some
problems; When I go to start the game it starts just fine, but when I
get to the main menu and hit new game the games loads about half-way
and then goes back to the main menu.

Any suggestions on how to fix this?

Thanks in advance,

Dan

Here are the specs on my coumper:

Tyan K8S
Windows XP Pro
Dual Opteron 1.6
512mb pc3200 ddr
2 30gb 15k HD
Radeon 8500

p.s

Sorry that i so heavly cross-posted, i just have no idea where to post
this. If youy have any sugestions, please sugest.