Close Winamp, then open this file in Notepad:
C:\Program Files\Winamp\Plugins\ml\ml_online.ini
(change the location if you've installed Winamp elsewhere).
Replace this line
ElementID11=-1
with
ElementID11=11000
ElementIcon11=11007
ElementSubbed11=1
ElementName11=Pandora
ElementURL11=http://www.pandora.com?cmd=mini
ElementID12=-1
In fact, you should find the last line that contains ElementID[number]=-1, replace it with the code above, where instead of 11 you write [number].
Save the file, then right-click on it in Explorer, select Properties and check "Read-only" in the Attributes section.
When you open Winamp again, go to Media Library/Online Services/Pandora. If you don't have a Pandora account, you should make one. You can close Media Library and still listen to the radio.