Preferences > File Types > Check Enqueue files on double-click in Window Explorer
Then it'll append the file to the current playlist and not start another instance of Winamp.
(Official) Winamp versions before 5 had no encoder, early Winamp 5 versions used an older Lame version that didn't have the ultra-slow mode.
Lame has 10 quality/speed modes, from q0 to q9, 0 being the slowest. At some point a new (superslow) q0 mode was added and all others were bumped up one number.
Extra High translates to q0, High to q2 (which was q1 in older versions), Normal to (I think) q5, and fast to q7.
However, that older Lame version had its own serious bugs and didn't produce the same quality as current ones (or 3.90.3), and the older encoder didn't support alt-presets.
Originally posted by DJ Egg
From the whatsnew.txt (About Winamp > Version History tab)
* upgraded MP3 encoding to LAME v3.96
* upgraded MP3 encoding to LAME v3.95.1
and before that it was v3.90.3
Actually it was the buggy Lame 3.93.