EasyBCD

EasyBCD is a utility that allows you to easily and quickly modify your Windows Vista startup settings to dual-boot other operating systems, such as Linux and Mac OS X.

That being said, these operating systems have to be installed in a separate hard disk partition or other unit. You just have to select the partition in which the operating system in question is stored, indicate which operating system will launch by default, and how many seconds that will pass before it starts.

You can also use EasyBCD to save a backup of Vista's startup or to save the previous one from Windows XP.

There are tons of advanced options that make this program easy to use for both novice and expert users.