What sound card?
I suggest, for starters looking at turning down the volume from within Windows, and possibly on WinAmp itself, especially if it is soley WinAmp that's giving you the trouble.
if the mp3's you are playing are at a low "kbps" then the sound distorting will happen. Does it happen when listening to CD's? If so as Big B said try turning the volume down winin windows...
Control Panel > Sounds And Audio Devices!