When setting up multiple operating systems, previously-installed systems can become inaccessible in the boot manager. It's easy to fix the boot manager so that both Windows 7 and the older operating system display correctly, and it should only take a few minutes of your time.
Problems with the Boot Manager
If you are setting up a dual-boot or multi-boot system and have Windows 7 installed first, problems can occur with the boot manager once older versions of Windows have been installed. This is especially prevalent when installing Windows XP, since the Windows XP boot manager may not recognize your installation of Windows 7 at all. The reason for this is the fact that unlike XP, Windows 7 does not use the boot.ini file as its boot manager. With other versions of Windows you would be able to edit boot.ini to add the new operating system to the boot order, but Windows 7 cannot be added this easily. This is because Windows 7 uses its own boot manager, so you will need to reinstall its boot manager so that it overwrites the one used by Windows XP. Fortunately, reinstalling the Windows 7 boot manager isn't difficult.
Using the Windows 7 Repair Function
One method of fixing the boot manager so that both Windows 7 and Windows XP are displayed and boot correctly is to use the Windows 7 installation DVD to access the Windows 7 repair function. You can access the repair function by placing the Windows 7 DVD into your computer's DVD drive and rebooting the computer. Allow the computer to boot from the DVD as though you were installing the operating system again, choosing the drive from your BIOS boot menu if necessary.
When the installation program launches, select your language options as you normally would and then click "Next" to advance. Choose the "Repair Your Computer" option from the menu, then select "Startup Repair" on the next screen. Follow the on-screen prompts to re-install the Windows 7 boot manager. Since Windows 7 can recognize Windows XP automatically, your Windows XP installation will be added to the Windows 7 boot manager automatically.
Using Third-Party Software
Another option that will allow you to add Windows 7 to the boot manager is to use a third-party BCD software solution. There are a number of BCD and bootloader programs which can be run from within Windows XP or Windows 7. Download and install the program, launch it, and create the boot options for any operating system that you have installed which doesn't currently appear in your boot menu.
Once you have made sure that all of your operating systems have been entered into the BCD software, locate the option to manage the bootloader and select "Reinstall Windows 7 Bootloader." Once this has been selected, you will simply need to click the button to rewrite the master boot record or MBR; this will overwrite the current master boot record with the information that you have entered and will also install the Windows 7 bootloader over the Windows XP version so that Windows 7 will boot correctly.