Do the popups from this have anything to do with the latest build of eJukebox?

Popups from this address: http://www.sqrsoft.com.ar/en/donate.html

The other major possibility is Winamp 5 Beta 2, which I've also recently downloaded.

The popup definitely IS only happening during my music playing.

EDIT - Never mind. I found the answer myself. Neither Winamp 5 nor eJ are to blame. It's the crossfading plugin--the one recommended on the plugins page.

Have you tried what they say at the bottom of the web page?

"To disable this window download the last plugin version from the SqrSoft web site and go to the plugin advanced configuration window to turn it off!"

That's how I figured out it wasn't Winamp.