G.10 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 re-enable Windows' default handling of tapping the <ALT> key by setting w32-pass-alt-to-system to a non-nil value.