How can i install two os on a single drive so that i could later safely remove one of them?

I would like to know is it possible to install two OS on a single HDD without having to split it in two different partitions so that i could boot up Vista and also be able to stay with XP at the same time, i'm afraid that if i'll install vista and xp on the same drive then one of the system folders could get modified replaced with a new fresh empty one or simply deleted. For instance when you install a new copy of XP without deleting the old one then during installation you get warned that the folder named ';Windows'; already exists so in order to keep your files from old windows and get a fresh install you rename the new windows folder to something else but can i somehow install two OS without having to split my drive? And also if there is no other way then can i merge my two hdd partitions back again?How can i install two os on a single drive so that i could later safely remove one of them?
You'll need to repartition the empty space on the drive to allow for XP to be installed. This can normally be done with Vista's partitioning tool relatively easily.