Right just had a look on the download site from microsoft and something that caught my eye was this:

In addition, once you install Windows Vista Beta 2 (or RC1) you cannot roll back to the previous operating system installation—you will either have to acquire and install the final released edition of Windows Vista or reinstall a previous edition of Windows.

So basically if it fucks up or your getting too many issues you cant do anything unless you wait for the final vista realease or you must have an xp disc? is that what its saying? if so then your screwed if you dont have an xp disc