Which servers are you trying to connect to?
You may have more luck using the Gamespyarcade servers as they alwys worked best when my son played the game when it first came out.
http://www.gamespyarcade.com/
I also seem to recall that the original game install needed a few downlaodable patches from the main site to cure the inevitable bugs so worth checking that out too. There will also be a longish wait to play some of the on-line games as your system will need to download updated maps and other details for the specific level being played on any server you are trying to connect to.
Out of interest does the game itself play ok on your own PC?
OK That's all I can think of for now but will check with my son next time I see him and see if he remebers exactly the step he used.