When you open these project/solution,
references to Outlook and Office will
be missing, and marked with a warning
symbol in the References folder
(found in the Solution Explorer).

You need to remove these references and
then add them again, by right-clicking
on the references folder and selecting
"Add Reference". In the dialog that will
appear, select the COM tab and then find
the Microsoft Office 10.0 Object Library
and the Microsoft Outlook 10.0 Object
Library (your version numbers may vary,
10.0 is from Office XP). 

Click "Select" to add these two libraries
to the lower list box and then OK to add
them as references to your project.

You will be prompted that interop assemblies
need to be created, and you should just
click Yes for both object libraries.