If you are not already running Vista Service Pack 1, you cannot install SP2 Beta. If you're running Vista and haven't yet installed SP1, you will need to do so before installing SP2 Beta. It would be nice if Microsoft provided a "combo" update so that any Vista user can install SP2 Beta regardless of whether they use SP1, but I imagine that would be a ridiculously huge download.
Microsoft actually gives you the choice to install Vista Service Pack 2 Beta from a standalone installer, an ISO disk image, or from Windows Update. To install from Windows Update, you will need to first download and install a script so Windows Update can "see" SP2 Beta, so to speak. For my testing, I went ahead and used the standalone 32-bit installer, a 388 MB file.