AIMP comes with an audio converter, but you will need the LAME codec to be able to convert files to the MP3 format.
First, download the codec from the following page: http://www.rarewares.org/mp3-lame-bundle.php
Extract the archive and copy the lame.exe and lame_enc.dll to the following folder:
C:\Program Files (x86)\AIMP3\System\Encoders
Now, start the Aimp: Audio Encoder from Start > All Programs > AIMP and choose the MP3 format from the Format drop menu. Press on the +(plus) sign to add files, choose the output directory and press on Start.