This type of error typically happens for a couple of reasons:
1) Your Serial Number has been upgraded and you now have a later version. If you would like for me to check the status of your Serial Number, please PM me (click my amanda.k user name, then "Send PM") and I'll let you know where it stands.
2) You have a later version of the program installed, such as 2011 or 2012 and are trying to activate it using your 2010 Serial Number. Each release has a different Serial Number and Product Key. Please double check all the information matches up with the download.