I sort of remember there was a way to enable the Alt-Graphics keyboard(s) on Windows.
This might be what you’re looking for: http://ohkine.tumblr.com/post/376098469/macintosh-keyboard-layouts-for-w...
The combination Ctrl+Alt on Windows is equivalent to AltGr. You need a keyboard driver that maps this combination, though. A lot of Windows keyboards, especially non Latin, use Ctrl+Alt (and Ctrl+Alt+Shift) for rare characters in a given writing system. You have to be careful when creating such keyboard layouts though, because you can run into interference from Office hotkey combinations.