First, go to Help -> Check for Updates...
If the problem persists, try Help -> Repair Adobe Reader Installation
The last resource would be uninstall the Adobe Reader XI, run Acrobat Cleaner (get it from http://labs.adobe.com/downloads/acrobatcleaner.html), and reinstall it.
While most installs, uninstalls, updates, and repair operations happen without incident, there are cases where a user may not be able to complete such tasks due to some registry or file conflict on the machine. This is particularly problematic when permissions set on registry entries or files prevent the successful installation of new installs and/or updates. The Adobe Reader and Acrobat Cleaner Tool is designed to fix such issues by cleaning up corrupted installations, including removing or fixing corrupted files, removing or changing permissions registry entries, etc. The tool provides options for removing problematic Acrobat items only while leaving Reader untouched and vice versa.