Remove the xp drive and set the Win 7 as the master. If the Win 7 drive is a Western Digital then leave the master/slave jumper off. Leave out the xp drive for now.
Follow the directions from here:http://www.ehow.com/how_4836283_repair-mbr-windows.html
Once Win 7 is booting correctly you can reinstall the old XP drive, setting jumpers on both drives as needed. You can then copy over any data and reformat the drive to your liking.