Howtogeek shows you how to manually repair Windows 7 boot loader problems

If you're having boot problems on your Windows PC, it's often helpful to repair the MBR (Master Boot Record) to restore the Windows 7 boot loader and you can do it easily from the Windows installation disc.
This is generally most useful if you've broken something and there's a boot loader error, or if you have made the mistake of installing an older version of Windows on the same PC that already has Windows 7 which wipes out the boot loader.
Note: If your PC starts booting into Windows but fails, you should probably try using Safe Mode instead.

  How to Manually Repair Windows 7 Boot Loader Problems