Are you pointing the compiler at the .map file? The .bsp file is the compiled map--aka the file trem uses to draw the map.
EDIT:
Oops...
For some reason the first time I read your post, I only saw the last three lines...
Weird.
Anyway, have you created "C:/Program Files/Tremulous/base/maps/"?
EDIT2:
Doh!!
You have...
Maybe move/delete the old bsp if it is there?