I'm running Windows 8.1 in Boot Camp under Mountain Lion on a MacBook Air. I've tried every combination of Command, Option, Control and Shift and I can't figure out how to produce the equivalent of the Windows Key. I'm also using Parallels to run the Boot Camp image of Win 8.1 in a window alongside other Mac apps, so I'm not sure if the Windows Key maps differently to the Mac keyboard under Parallels versus Boot Camp alone.
Is this just a problem with Windows 8? With Windows 7 running in Boot Camp, the ⌘ Cmd by itself acts as the ⊞ Win. If I tap just the ⌘ Cmd, the Start Menu pops up. If I type ⌘ Cmd-L, my screen will lock.
Within Windows, you may want to check "Apple Software Update" to see if any new drivers are available. Also check the Keyboard tab in the Boot Camp control panel. In the Boot Camp control panel there is also a Help button, and one of the topics mentions that you can use the "Add or Remove Programs" control panel to remove the Boot Camp drivers if you want to reinstall to ensure a fresh installation.