http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/windowing/windows/windowreference/windowfunctions/getwindowtext.asp
http://msdn2.microsoft.com/en-us/library/ms633584.aspx
Ну и общая инфа по http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/buttons/buttons.asp (для чекбокса конкретно http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/buttons/buttonreference/buttonmessages/bm_getcheck.asp) и по http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/editcontrols/editcontrols.asp (конкретней - http://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/editcontrols/editcontrolreference/editcontrolmessages/em_getline.asp)
Разбирайся
