C# search directory path

I am working on setting up a C# 2010 application that will search directory file paths to see when there has been a files added to the directory since the last time the directory was searched. I basically want to look for the following:
1. files that end with *.xls or *.xlsx.
2. I will look for files in the 'highest' level directory path and serach for new files that have been added since  the last time the search was made.

Thus I am hoping  you can tell me how to accomplish this goal and/or point me to referneces that will let me how to accomplish this goal.