Why won't my Team Fortress 2 game wont work?

I recently bought Team Fortress 2 and installed it on my Windows XP Media Center Edition 2005. I followed all the instructions donwloading it yet it still has several problems. When I tired to load a sever it wont load the final bar, there have also been times when the screen flashes wildly, I can also here nosies of people fighting at the loading screen. The game also sometimes freezers at the opening credits. In some rare occasion the game connects but the words low OS paged pool memory flash in red. Can anyone tell me how to fix this, I really want to play this game.