What are you using to deploy the project? If you create a setup and deployment project you control the install directory there (though if you dont want the end user to be able to change then you have to do some changes to the setup UI too - though i always recommend you let a user choose).
If you want to fix the path for some reason - it rings warning bells. Why are you hardcoding paths? They should be relative to the application. If you need to hardcode to get to something then you should use isolated storage instead (for example if you are storing settings or some information that the user configures).
If it is to get to something in your app then you should use a relative path from your app directory (if you need to pass the full path to something you can get the app directory path at runtime).
Hope i have helped an not confused or made things worse