xxx.bkf is the correct file extension for backup files created by WinXP. What does Win 7 use for a file extension for backup files ? Knowing MS it might be completely different AND use a different file system. They like to pull tricks like that to render their older OSs obsolete so as to FORCE people to BUY the newer ones. e.g. Win XP could not open Win 98 SE wordpad files and Win 98 SE could not open Win XP wordpad files unless you used the rtf, (Rich Texture), file extension which had lousy formatting capabilities.
Unless Win 7 can somehow be tricked into accepting an XP backup file for restoration, I think what you are going to have to do is hook the drive up to an XP machine and do your file restore that way. Since the drive is one Terabyte and you only have 400 Gig of files on it, you can do the restore to the empty space on the drive, then once that is done, disconnect from the XP machine and hook it back up to your Win 7 machine, at wich point you should be able to : 1) access your files, 2) Delete the XP backup, 3) Create a Win 7 backup in its place.