If it starts with the Windows 7 DVD or Repair CD in the drive but not without it, then the boot files need repairing or rewriting.
But first you have to make sure the correct partition is marked Active so the Repair utility knows where to write the System boot files.
To do this boot free Partition Wizard bootable CD
make sure the Windows 7 partition or its 100mb System Reserved partition (preferred if you have it) is marked Active. If not, rightclick to Modify>Set to Active, OK.
Next click on the Windows 7 HD to highlight it, from DIsk tab select Rebuild MBR, Apply, reboot. This may preclude the need to run 3 Startup Repairs. If not boot the Windows 7 DVD or Repair console to run Startup Repair - Run 3 Separate Time.