On Windows 7, Microsoft Office 2010 and 2013 will take over the file association
for text-like files. The Office installer sets the default handler to use Office
XML Handler for anything that was associated with Notepad++. The fix
is to delete the registry key HKEY_CLASSES_ROOT\Notepad++_file and start
Notepad++ with admin privileges. Go into the settings, remove and re-add one
file association, and quit Notepad++. The file associations should be fixed, no
reboot required. Fixing the icons may require a reboot requires restarting
the explorer.exe process. Open a Task Manager window (Ctrl+Shift+Esc),
end the explorer.exe process, and in the File menu, choose “New Task”, type
explorer.exe and bingo, fixed.
Source: http://sourceforge.net/p/notepad-plus/discussion/331754/thread/0a714707