Just remember (unless you know the "secret"), once you install Vista, your old system's (XP) serial number is forfeited. You can't reinstall XP.
Actually once you own the XP disk and the key code you can uninstall and reinstall it as many times as you wish even after installing Vista.
It would be harder to get XP back on your machine IF you installed Vista by performing an upgrade from XP and kept all of your programs and files then unfortunately there is no way of returning to XP short of backing up your data and formating the partition.
BUT
If you installed by booting from the disc or doing a clean install (and got
rid of all your Windows XP files) then you can return to XP with all of your files intact.
But I'm sure there are some guys stuck in a room right now, writing computer programs to fix it and am waiting patiently for them to finish.