Additional things that would help:
1. Your OS
2. Where did you get the download?
3. Was the download for the same product: Std, Pro, Full or Upgrade.
If it was an upgrade, then it may be the S/N requested is the previous version. However, I don't think the upgrade version can be downloaded. So it may be that your S/N is for an upgrade and the download is not.
Did you not keep a copy of the original download for future use?