This is why I back up everything including CD-Keys. Whenever I download something, I save the CD-Key to a text file called "program_name_cd-key.txt". In this file is the copied and pasted cd-key from the download. Yes, this information is on the web, but it's easier to get it when it's local.
This file is burned to the DVD with the downloads just in case I have to do a new install. When it comes time to install, all I do is copy and paste in the cd-key from the text file into the field for the cd-key in the installer. This saves time and errors with typing in the cd-key.
John