If you install Windows 7 as second operating system along with Windows Vista or Windows XP the boot menu will automatically add new OS option in existing boot menu option. However if you have already installed Windows 7 and attempting to install Windows XP you’ll overwrite the MBR with one that doesn’t recognize the Windows 7 boot loader.
If you are under similar situation and looking for the way to fix and get back Windows 7 option in boot menu then follow below simple steps.
Open a Command Prompt window in the older operating system and run the following command from the Windows 7 DVD, substituting the letter of your DVD drive for d here:
d:\boot\ bootsect.exe /nt60 all
When you restart, you should see the Windows 7 menu. To restore the menu entry for your earlier version of Windows, open an elevated Command Prompt window and type this command:
bcdedit /create {ntldr} –d “Menu description goes here”
Substitute your own description for the placeholder text, being sure to include the quotation marks. The next time you start your computer, the menus should appear as you intended.
source: Technet
