No, Mac and Windows uses different file(s) even though the program is on the same CD for both systems.
Blizzard has for as long as I can remember offered both PC/MAC versions on their products which made them huge hits in the Mac market.
So again no, not possible as the PC requires a vast amount of .dll's and reg settings in order for it to work when installed normally.
If you want to use it from a external HD you can do that but you will can to install it from your windows machine and target that drive which forces all the reg settings to point to that external drive.