If that is 0 (zero), then I have a problem with your statement "menu 'Window -> 0' is channel" as my mIRC channel (and pm) windows start at 1.
Alt + 0 does nothing on my system.
If that is O (Oh) then I see that as for putting the windows into order.
Alt + O brings up the mIRC Options window.
Additionally, are you referring to the left or right Alt key?
If it is the right Alt key, then this discussion
may be relevant.