I am going to disobey my better judgemet and make a suggestion without having more information: Usually when I have problems of an odd or serious nature (which yours falls into both of these), I delete my winamp.ini preferences file.
Yes, I know. It sucks to lose your prefs. But there is a good chance, in my opinion, that it fix the problem.
If nothing else, try renaming winamp.ini and seeing if it fixes the problem. If not, then rename it back and switch back to 2.4. Nothing lost.