did you just install the game.   or has it worked before

Maybe try a fixed EXE no cd/dvd crack...

Quoted: Quoted: Maybe try a fixed EXE no cd/dvd crack... I'm fairly computer illiterate, what does that mean?  I went and traded the game for another copy, still have the same problem.  I can play on multiplayer just fine, but I get the error message whenever I try to play single player.  I get the message about inserting the correct CD.

You can get a cracked exe file that replaces the original one for the game, that removes the CD checks and copy protection.  Call of Duty is exceptionally easy, because the multiplayer exe is a different file than the singleplayer exe file - so you can crack the single player, but leave the working multiplayer alone.  

Get this file if you are using 1.0 of Call of Duty 2 -

%20No-CD/Fixed%20EXE]m0005.gamecopyworld.com/games/pc_call_of_duty_2.shtml#Call%20of%20Duty%202%20v1.0%20[ENGLISH]%20No-CD/Fixed%20EXE

and get this one if you installed the 1.01 patch for the game -

%20No-CD/Fixed%20EXE]m0005.gamecopyworld.com/games/pc_call_of_duty_2.shtml#Call%20of%20Duty%202%20v1.01%20(Intel)%20[MULTI]%20No-CD/Fixed%20EXE

Make sure to only overwrite the single player file with the cracked one, not the multiplayer - or else the multiplayer probably wont work.