Windows Explorer already has it built in. Just go to details view, right click one of the column headers and turn on the "bitrate" option. When you select the "More..." option, you can see that Explorer can display 38 diferent file parameters (not just Name, Type, Size and Date which are default). e.g. In addition to bitrate, you can display track number, year, artist, album title, genre, duration etc.
This is true for 32-bit XP Pro SP2 - not sure if this applies to Vista-64...