I'll try to help, but I'm no expert. First off, what version of the game do you have 1.4 or 1.5. If you don't have 1.5 I strongly suggest you purchase it. It's only $10.00 to d/l, and it patches the game to v1.5 as well as adds U boat missions. Not to mention the fact that all the major mods are pretty much geared for 1.5.
Next, what are your system specs? Is your rig up to the task of running the game? I know these sound like stupid questions, but we need to know the basics