As discussed here, previous versions of throttle.exe would not succeed in throttling a P3B-F (and certain other mainboards based on the PIIX4 southbridge).

Jeff Leyda, the author of throttle.exe, has taken the time to update his program to support these mainboards. The update can be found here: http://www.waste.org/~winkles/throttle/THROTTLE_cc_en.zip