Had same issue. Got it fixed:
Reinstalling the mp3el.exe (direct, NOT within Wavepad) works IF: You let Vista tell you it is installed (a LIE) then Vista after a bit of delay pops up a window asking if you trust the file, answer yes and you get a pop up that lets you run the thing again as administrator. Then it tells you it is installed and this time it's TRUE. :-| hehehe
Sheesh, Microsoft. But anyway, it now works!