Somehow, PDFs have become associated with Media center as the default application for opening them.
Right click a PDfFand choose Properties.
To the right of Opens with, there is a Change button. Click it and select Adobe Reader from the list of programs.
Click OK, Check the box to always use the selected program to open this type of file (If it's clickable), then click Apply and OK.
A reboot may be necessary.
That's a great tip, got everything fixed!