You could also boot to the Install DVD, go to repair and to the command prompt. Type in the following and wait for a response.
It will search for any Windows Vista or Win 7 installs not currently on the boot menu. Then it will ask if you want to add the ones it finds. If you allow, it will put the other install in the boot menu. You should then be able to reboot and have both options available.
The EasyBCD method suggested by Casca also works well, but I would suggest you register in their forum
and download the version 2.0 beta. It works better with Win 7...