First make sure that the first beta of the Windows Vista Service Pack 2 is completely uninstalled from the computer system as the new beta cannot be installed over the old one. Once the first beta of Windows Vista Service Pack 2 has been uninstalled you can go ahead and add the following Registry keys to the Windows Registry: The easiest way to do this is to copy and paste both lines into a new text documents and rename it so that the extension changes from txt to reg.

reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\VistaSP2 /f
reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\VistaSP2 /v Beta2 /t REG_SZ /d DCA54ECC-362A-4B4D-B62B-22780E839A7E /f

A double-click will execute the file and add both lines to the Windows Registry. You can try and check Windows Update right away to see if Beta 2 shows up. It might take some time and you might need to reboot the computer after applying the Registry patches before you can see the new beta release.