I don't know why this required a new thread (could've kept it as a reply to the previous topic), but anyhow . . .
At this point, it could be several things causing trouble. What is the error message, exactly? Is the Dedicated Server setting in the Host screen set to No? It's important that it's set to No.
Maybe another program that's running, is interfering with the game and causing it to crash? What version of Windows are you using? Is it XP? Try using the program EndItAll2
to shut down any unnecessary programs. Shut down your anti-virus, firewall, etc. temporarily and see if it helps any.
What video and sound card do you have? Do you have the latest software drivers for them? I've heard stories about people having trouble getting the game to work properly, with the latest nVidia cards and the latest Detonator drivers. Don't know how much truth there is to it, though.