Apple Script and Startup Disk

I currently have a script as follows:

set theFolder to "Macintosh HD:System Folder:preferences:"

Now not everyone has their startup disk titled "Macintosh HD"

does anyone know how to do it so that no matter what the name of the startup disk is, it will set "theFolder" to the path to preferences?

I though it involved "startup disk" but that didn't work when I replaced Macintosh HD" with "startup disk"