Your key-combinations (where Alt and A are both pressed at the same time) work, even if a single keypress on the Alt prevents further input. A keypress on Alt should give you the option to browse the menu. Windows doesn't capture the key, but the focus. The second keystroke then determines what menu to open. That's two seperate keystrokes. Your key-combination is Alt-A, and the A is typed while the Alt-key is down. That gives different behaviour on Windows - it first checks if there's a shortcut-key defined (e.g., the letter right after the "&"-char in the caption of a button) These two behaviors shouldn't bite each other. If your end-user is like me, he'll be using the Alt-key often enough to navigate the menu :)
I are Troll :)