Cannot Mount The Backup

Hi everyone. I have Windows XP Pro 32 bit. My backups seem to be completeing just fine and have been for quite a few months. I find that I need to restore a few files on my computer, and I'm unable to open any of my backups. I get the error "Cannot mount the backup." I do not have any drives mapped. No Z or any other..just my C drive and my DVD which is D. I've done a search, and everything points at Windows 7 or mapping to Z...but I have neither of those. I've also tried restoring my computer using System Restore to 5 weeks ago..just in case it was some update. That didn't help either. Does anyone have any idea what may be wrong?

Thanks for any help!

Dave