Here is the simple tutorial that tells you how to dual-boot windows XP and windows 7 when you first installed Windows-7 and then XP. As if you would do it the system would automatically get booted to XP and won't show windows 7. Just follow the simple steps to get both the systems back-
Using the same procedure you could also Dual Boot into Linux and Vista..or any other OS too..
I have dual-booted my PC using windows 7 and Ubuntu. I will tell the tutorial later on. If any problem persists please comment on the post or mail me at firstname.lastname@example.org.