It is a typical situation. You installed Ubuntu or Windows XP over Windows 7, and now you can boot only the last installed OS. You wish to get back the multi boot menu that lets you boot all OSes

The first step is to restore the Windows 7 bootloader

Boot off the Windows 7 installation disc, click through all options till you get an option “Repair your computer” in bottom left. Choose the correct Windows 7 partition and click Next.

Now you get the System recovery Command Prompt

To restore the Master Boot Record with Windows 7 entry:

bootrec /fixmbr

 

If this fails to fix your issue, you can write a new boot sector to System partition with:

bootrec /fixboot

 

All the options supported by bootrec can be viewed with:

bootrec /?

 

Recover Windows 7 Bootloader after installing Windows XP on top of Windows 7:

bootrec /nt60 all