If you have a version of Vista that comes with the downgrade rights (the home versions don't), then you can switch from Vista to XP and could later switch back from XP to Vista. My understanding of the license is that only one can be on your system at one time; you must uninstall Vista before installing XP, and vice versa.
Refurbished PC's are a good way to get XP with a computer thrown in. You can also buy OEM copies of XP for $80-$90 last time I checked (try newegg.com). The difference between an OEM license and a full retail license is that the OEM license is such that once you've installed and activated your copy of XP on a machine, the license is married to that machine. With a full retail license, you can uninstall XP from the machine you have it on, and then install it on a different pc, i.e. you can move it from one machine to another.
Steve