Try adding Windows 7 from XP using the steps at the end of Method Two here: Dual Boot Installation with Windows 7 and XP
If this fails, burn a System Repair Disc
on same bit-version machine, boot to follow the steps given earlier.
Another method is to use free Partition Wizard bootable CD
to mark Windows 7 partition Active, then click on it's HD to highlight it, from Disk tab select Rebuild MBR, Apply steps, reboot.