Writing to the file system from a service

Question

I have working code for an application that uses a configuration file. Periodically the application does work and saves updated information to the configuration file. All is well when I use a conventional console project for my application. When I turn into a service, I have problems. With hard coding of directory paths I can make it all work, but when it comes to deployment it looks like all the pieces are there but they don't quite hang together. In my setup project I can create registry entries to be set on installation but I can't seem to create a registry entry to point at a location I can use to write file to (without hardcoding the registry entry). In my setup project I can put my original config file into a deployment directory in the file system but not one I can point at without hardcoding the registry entry. I find that I can't overwrite the configuration file I deploy, so I've set my app to read the original config and then write to a new file name - that seems to work OK, but if there's a way round this I'd be interested. I've found (by experiment - as I can't find guidance on which bits of the file system a service/deployed program can write to/read from) that I can read files and write to non-deployed files in the application data area. But I don't seem to be able to refer to the same application data area in code and at deployment time (e.g. I can get at user in one and all users in the other). Can anyone help? My requirement is: 1 write a config file with the installation whose path I can put into the registry at deployment time 2 read and rewrite either that file or one with a new name at run time from my service (using the location in the registry) (i.e. I'm not attached to Application Data areas - although that seems like the logical place to do this from - I just want somewhere in the file system that I can deploy and write to!) And of course I want to avoid hardcoding - i.e. do it properly!