Repair the MBR to restore Windows 7 to your Multiboot Options.

Each time we install a version of Windows, it rewrites the MBR to call its own boot loader. If we install Windows 7 (or Windows Vista) as a second operating system on a PC where Windows XP is already installed, the Windows boot menu incorporates the options from the older boot menu. But if you install a fresh copy of Windows XP (or Windows Server 2003) on a system that is already running Windows 7, you’ll overwrite the MBR with one that doesn’t recognize the Windows 7 boot loader. To repair the damage, 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.

An even easier solution is to use one of the boot-editing utilities we highlight in this section. Both VistaBootPRO and EasyBCD run on Windows XP, Windows Vista, Windows 7, and Windows Server 2003 or 2008, and they can be used to switch quickly from a Windows XP–style boot loader to its Windows 7 counterpart and back again.

Source:Microsoft websites