You should fix a set of coordinates, latitude and longitude which is your specific point. You have the position changed event for the geolocator which you can handle.
like here:: http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj247548(v=vs.105).aspx
Once you have that working, you can calculate distance between two locations as shown here,
Calculating Distance between two Latitude and Longitude GeoCoordinates
And you can show any pop up on the screen if you find that the distance is greater than or equal to 1km. Hope that helps.