How can I find the directory my program was installed to?

Let's say a user installed it in C:\MyApp
My program then puts the Excel files is uses in there aswell.

C:\MyApp\MyApp.exe
C:\MyApp\MyExcelFile1.xls
C:\MyApp\MyExcelFile2.xls

I'm reading the .xls files from MyApp.exe so I'ld like to know the location of the .xls files.

To do this I've found: Application.StartupPath
but when a user copies my application to run from somewhere else the startuppath doesn't help me finding my .xls files, they are still where the application was installed.

Anyone?