Because of the JavaScript sandbox, you can't do it without a middle layer requiring elevated security permissions, such as a Netscape-style browser plug-in (widely supported), ActiveX control (pretty much IE-only), or .Net control (I assume that's possible; again probably IE-only). In each case, the JavaScript would talk to the control, which would in turn make the USER32 call for you.

None of that will work without the user having granted your application elevated permissions, but I'm guessing as you're requiring Windows, this is for some kind of intranet application where that may be possible.