I just got a new PC and basically I don’t want to get in the situation I was in with my last one. I started putting mp3s in folders based on artist. That way, once I have several hundred artists on my computer, it would be fairly easy to pick out ~10 of them to listen to and add to a PL; however, I can’t seem to select multiple folders at once when adding to PL, so adding individual folders manually ten+ times gets tedious. Also, I can’t seem to add a folder that contains folders within, such as, say, a “Country” folder with “Alabama” and “Brooks and Dunn” folders within (yeeeehaw). Thanks for the help
The last version of Winamp that I used was 2.8, but even then it was pretty well integrated with Windows Explorer. If the Winamp developers have maintained that functionality, then you should be able to drag and drop folders from any Explorer window directly onto the Winamp playlist window, and whatever supported filetypes within the selected folders (including subdirectories) will be added to the Winamp playlist automatically.
If drag and drop isn’t your thing, Winamp’s preferences dialog box allows you to add context menu items to the Windows registry, so that right-clicking any folder in an Explorer window will show an entry labeled “Enqueue in Winamp”, which you may then select to add all supported filetypes within the highlighted folder(s).
If that doesn’t work with newer versions, can get 2.8 at oldversion.com
Love that site, and 2.8 
Here’s what I do in Winamp 5 (setup to look and act like Winamp 2):
I click on the ad file button, and then I drag the folders from the window that pops up into the playlist. And then, I hit cancel to make the window go away. Yeah, I’m sure it violates some user-interface style code, but it is convenient.
I believe Winamp 2.x will freak out if you drag stuff from the “add file” dialog into the playlist and then close the dialog without selecting a song the normal way. It’s been a while, so I am not sure.
Drag-and-drop multiple directories from Windows Explorer works fine with v. 5.02
Alternatively, download iTunes and get it to do it for you - it’s an automatic feature.