Can XP get started?

If so, you can try use BCD to recreate start disk. The problem maybe XP and Win7 uses different MBR where XP can't understand Win7.

 

On the contrary, if you can recreate with BCD a Win7 start up MBR, it should be able (by adding one entry in BCD) to see XP.

Another methods is to use ... Linux or other live CD to reset Active partition to Win7 partition and see if it works. I know Mac OS can do this easily with "fdisk"