A specific problem with altering the Mac command-key layout to behave like Windows
I don't think you can use alt + shift for anything by itself.  It is used for all kinds of special characters in combination with a letter key.

How Mac OS X lets you toggle between different international language keyboard layouts



How Mac OS X uses the "Option" key as a "dead key" to enable typing diacriticals for Western European languages in the US keyboard layout.



Here is where a conflict may arise.