Lifehacker - How to Dual Boot Windows 7 with XP or Vista - How To
First result in Google.
I have a dual boot running on my system (technically a tri-boot with XP/Vista/7 until I remove XP from here). Install Vista first, t hen install 7 and 7 will automatically become the default bootloader and add both OS's to the bootloader. That's basically it; just make sure you have enough room on each OS's partition you make.