is your acrobat pro xi an upgrade version?
if it is, that's not going to work unless you have a qualifying stand-alone acrobat version. you can't upgrade from a cs product (that contains acrobat) to the stand-alone acrobat.
if it is not, (i'm not sure this is worthwhile but) you could uninstall your cs installation, run the cs cleaner (http://www.adobe.com/support/contact/cscleanertool.html), reboot and then install acrobat xi followed by installing (the non-acrobat part) of your cs