When connecting to the Gamespy servers, if I use an already-existing Gamespy ID, it says I cannot connect. However, if I create a new account, it works and I can play online. Has anyone else had this problem and found a solution for it?

It might be down to the Gamespy ID being already in use, but I don't really see the issue if you can manage to actually play online if you use a new account. I mean, at least you can play online. Could you be a little more detailed with what the issue is?
Personally, I've never tried to play online at all or have a Gamespy ID, so I'd just have to make a new one.

I have the same problem. I recently got onto TElstra Bigpond cable and they gave me a netgear router model CG814WG. Ever since I have had this problem. I found this on another site "Ok, what I have learned since is that it is definitely a router related issue. Something to do with previously created accounts login attempts. Apparently when you try to log in to a previously created account your machine sends it's info/identity to gamespy, gamespy sends it back for verification, then it is sent back again for the actual "log on". When a router is used there is a small delay of possibly a second or too (maybe even less) and the request times out. Unfortunately this is not necessarily the routers fault or your machines fault, but the fault of the software programmers who created the game. Apparently it was an oversight in their error when they designed the gamespy log in system for this game, not allowing sufficient time for the echo request or whatever you would call it to return to gamespy for verification. Not much you can do about it other than connect without a router. The only other solution is to create a new game ID every time you play since there is nothing to verify with new accounts and they don't have to make the round trip twice to log on."
Can anyone suggest a workaround. Would be much appreciated....