[How often do you reinstall?]

My own personal system had to have XP reinstalled once. Of the 500 or so I have at work, we reimage from a standard base as a last resort and is very infrequent.

[back to xp]

Microsoft has never supported a downgrade path. As with any version of Windows, if you performed an upgrade and chose to save your previous OS then you will have the option to uninstall. If you just got click happy and zipped through the install of Vista beta without backing up your previous install, your only option is a complete reinstall from scratch.