There's a bunch of new shortcuts for Win 8 that involve pressing the Windows Key in combination with another key. OS X will want to intercept these as a Command Key combo on the OS X side. To use these commands, hold [fn] plus whatever other keys are required. So to pull up the "charm" bar...
Windows Key + C
fn + command + C
EDIT: This is a Parallels default configuration, but you can change these around yourself from the Parallels Desktop > Preferences menu. Select your virtual machine from the list on the left and you can turn on/off any of the bindings at will. Be aware that this may force you to become comfortable switching back and forth between Windows and OSX shortcuts for things like cut and paste.