If you boot OS's separately via BIOS then each HD with an OS needs to be System Active in DIsk Mgmt in order to boot.
The exception is if you have a System REserved boot partition for Win7, then it must remain active.
Please try marking your DISK1 XP active to make sure it can boot now. Or if you no longer need two XP's, if it isn't marked Active you can delete the OS in Disk Mgmt and repartition the HD for storage or other purposes.