AH.9 Using the System Menu on Windows

Emacs compiled as a native Windows application normally turns off the Windows feature that tapping the ALT key invokes the Windows menu. The reason is that the ALT also serves as META in Emacs. When using Emacs, users often press the META key temporarily and then change their minds; if this has the effect of bringing up the Windows menu, it alters the meaning of subsequent commands. Many users find this frustrating.

You can reenable Windows's default handling of tapping the ALT key by setting w32-pass-alt-to-system to a non-nil value.