Add track # to file names

In an attempt to clean up my 15k+ MP3 collection I've spent the past few months deleting all the numbers from the start of the file name. I now realize this was a mistake because the players I use most often (wmp and my PSP) won't sort by the track number in the tag. I would like to find a fast way to add the track numbers to the front of the file names so that it would look something like this:
01 - Introduction.mp3

Can anyone help me to create an action that will transfer the track number info to the file name of the mp3's? Much thanks.

Action: Format value
Field: _FILENAME
Format string: $num(%track%,2) - %_filename%

You are the man! Thanks for this!