Lost cd key

[Bluegrass]

Please Help.

I bought Halo Combat Evolved for my son for Christmas. He installed it on to his computer. Now, we can find the disc but cannot find the CD Jewel Case with the cd key on it. Can anyone tell me if the cd key would be in a file on our computer. I am worried that I have thrown the Jewel case out with all the used Christmas wrapping paper. If the cd key isnt in a folder or file somewhere - do I contact Microsoft for a new key and will they give it to me??? The cd was bought on Ebay.

Thanks