Acrobat not loading Core.DLL

I am using a computer running Windows7 (64 bit) and cannot open or run Adobe Reader.  Either 9.5, 10, or version 11.  After a reinstallation of any of these versions I get a message that "Acrobat failed to load Core.DLL" and then a windows that says it cannot open in "Protected" mode. We have not had any problems  ( for years) with any version of Reader but the train has come off the rails.  I need help badly.  Help!