Mark Partition Active
the Windows 7 100mb System Reserved boot partition (preferred if you have it) or Windows 7 partition.
Run Startup Repair - Run up to 3 Separate Times
If it hasn't been too messed up then it should start Windows 7 and add XP to the Boot Menu. If not add XP using EasyBCD from Windows 7.
If Windows 7 won't start with SysReserved marked Active, move the Active flag to C to try repairs again.
If that fails, use Easy BCD from XP to transfer the System boot to Windows 7: Bootmgr - Move to C:\ with EasyBCD - Windows 7 Help Forums
. But this may not rewrite the Repair My Computer link to F8 Advanced Boot Options,
so if not run Startup Repair a few times until the link appears.