you can hook anything. I normally would just load in Olly and check the Call Stack, since you should be in the message loop the call before GetMessage or PeekMessage should be winMain, but since CA is packed all I can think of is Possibly hooking get message, checking EBP, before its set to ESP. Then just dumping the area around the address, and look for the spot you want to mess with or whatever you want to do.
EDIT: Oh you want to get keyboard & mouse input, the hooking GetMessageW should be perfect for that. you can read any input message and use it to control ur code.