Also easily undone ..
Launch MSCONFIG by from the Run menu.
Click on the Tools tab. Scroll down till you find "Disable UAP" (this should probably change to UAC in next Vista beta builds and in the RTM version). Click on that line.
Press the Launch button.
A CMD window will open. When the command is done, you can close the window.
Close MSCONFIG. You need to reboot the computer for changes to apply.
You can re-enable UAC by selecting the "Enable UAP" line and then clicking on the Launch button.