You can do this by using registry hacks.But it is only tested in Windows 8 consumer preview.May change in Windows 8 final version.
Switch to the desktop and open run.
Then type regedit.
Go to RPEnabled.
Change its value from 1 to 0.
Then you are done.This will move you to the start menu from the start screen that you are used to.But this may cause you to boot the system.Thank you.