Cant load a new game :'(

Hi guys, i have some really bad problems with doom 3.
after installing the game i start the game, so i click on new game and then after a few seconds (when its loading) it goes back to the title screen, i cant load any maps!!

i just got my brand new Dell, P4 2,8 ghz with 512mb ram and a ATI radeon 9800 pro, what the hell is going on :( :(

plees help me out!