OK, If this does not work, you may be faced with re-installing windows vista, as the xp installation was the only active file system at the time of defragging the other drive, so it didn't know what vista files were "protected system files" and most likely moved them to another part of the drive, messing up vista's boot sector/mbr or even vista's MFT "master file table" at the same time...

If the bootrec.exe fails to restore the partition's boot sector, master boot record or whatnot/ unable to boot to vista and you have important files that you need to backup, you may be able to browse/explore the vista partition for the important files via windows xp.. If you are unable to browse the partition via XP, there is a program from Acronis called "Disk Director Suite 10" This application allows you to scan a hard disk for data and restore it...

It costs money but it works! A good way to find out if that application will do what you need, check out the free demo version, it allows you to scan the hard drive, and look at the recoverable data, to see if it finds what you need... You can then buy the program either from a store, or pay for and download the full version from acronis.

I have used it to restore a deleted and formatted primary boot partition; that contained the entire drive's partition information "multiple partitions on a RAID volume containing photo's and another OS's temporary files/scratch disk and user profiles" to bootable status, with all the data intact...