The Windows Vista Team published an article on dual-booting Windows Vsta

I've noticed that there are many questions in the community about dual-booting Windows Vista. This is a very common scenario at Microsoft and as such, I figured I'd walk you through our typical dual-boot installation procedure via the corporate network. Many of my colleagues choose to run both operating systems simultaneously (not me -- I'm all Windows Vista, all the time) by partitioning their hard drives and running a separate OS on each partition. They do this for a variety of reasons, but in many cases it's so they can test new builds of Windows Vista while retaining Windows XP (or another OS) on another partition. This will allows them the flexibility to perform build-to-build upgrades more easily while retaining the original OS as an alternative should there be a blocking bug in the Windows Vista build.

Exercising the Dual-Boot Option with Windows Vista