Slow Me Down plugin will let you slow down or even speed up the music by stretching it between -50% to 100%. Without changing the pitch!
You can also fine-tune the pitch in cents (100ths of a semi-tone) to suit your instrument.
Note 1: No Winamp DSP plugin works when playing CD's, only when playing mp3, wave files etc.
Note 2: This plugin does not work with Winamp 3 or higher - only works with Winamp 2.