Battlefield Vietnam CD?

Hello, today I was cleaning around my computer and found the disc to Battlefield Vietnam (And no I do not mean the BadCommpany 2 expansion) , and decided to load it up and have some fun. So I got a couple of my friends who had the game and got ready to get in a match with them. It then said "error, invalid CD key" or something like that. So I told them to wait and grabbed my case and looked all over for the CD Key... but came out with nothing. So then I asked my friends if they had any extra's (which they didn't) So I was angry because I wasn't going to download an Illegal Possibly virus infected Keygen like one of my friends sugested. So now I'm stuck and in need of help. I'm just asking if theres any possible way I cant get a CD key LEGALLY without having to track down a new copy of a game that is'nt anywhere to buy on the internet? Thanks! (P.S.) Sorry I put it under Battlefield 1942, I just put it there because there was no option for Battlefield Vietnam, and I thought they used a simmilar engine.