anyone know why everytime i install unreal tournament 2004,it says cd key is invailed when i just bought this game not while ago anyone know how to fix this problem?

When is it saying cd key invalid? If it is when you try to connect to a multiplayer server then this is why. As GOG distribute a standard installer, everyone gets the same key on installation. If you want to go online then you can raise a support request to GOG asking for your unique key (the earlier released games on GOG need you to send in support requests, but newer released ones provide your key in your my account page) and they'll send you your key. I think for UT2K4 it is a registry file that you can run.

Here is the support form for UT2K4: http://www.gog.com/support/contact/technical_issues_with_games/unreal_tournament_2004_ece

Choose the multiplayer key request option for problem type.

Hope this helps.