Problem with XP Pro Backup

We have several computers for a photography business. I am have been using external hard drives to store duplicate files. On one I tried using the Backup on XP pro and now I am having trouble restoring specific files form it. I reinstalled the external HD that I had backed up to, but the Internal Hd has been changed quite a bit since. It wont recognize the any of the backup drive although it describes the location and size of a file. I have even been trying to do a complete backup to another HD as an alternate location. Is there some tweak software or something stupid that I am missing.confused