Adobe Reader XI protected mode problem

I downloaded Reader XI and went to open a pdf that previously worked with Reader.  But Adobe Reader XI won't open (says problem with "protected mode").  I can't open any pdf's either on my computer or on the internet at trusted sights.  Troubleshooting takes me to a page on fixes for Reader X. Not helpful!  Any ideas?  (I uninstalled and reinstalled and still won't work.)