Will Microsoft replace an invalid product key?

I decided to reinstall XP on my computer (I have a mac but I'm using Parallels) because I had a virus and the virus prevented me from downloading any antivirus. When I reinstalled it, it asked for my product key. I bought my computer from eBay and when I bought it, the sellers installed Parallels for me so I had no trouble accessing it previously before. There was no COA on the box, so I tried looking in the .txt file that tells you your product key. I entered but I got the message that the product key is invalid. If I called them, would they send a replacement product key or something?