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.