MidiMoto is correct. Vista, like Windows 2000 and Windows XP, can be installed and dual boot with any other Windows OS.

However, this is the first time that the process has been destructive if you installed onto the same partition. I imagine this is probably due to the fact the installer isn't tuned yet. If you installed Beta 2, there were quite a few problems with the new installer that have been overcome with Pre-RC1.

If you installed Windows Vista onto c:\, Windows XP wont be bootable. It might work to run a Windows repair, but I wouldn't advise this course of action at all. If you re-installed Windows XP without a format, you'd avoid losing any data, but your filesystem gets progressively messier when you install Windows more than once without formatting.