After starting to convert my 400 CD collection from MP3 to AAC for iTunes 4, Jason mentioned that windows does not have a good player for AAC files, if any. So if you know of one, please post a comment. If I find one, I'll update this entry.
Update: Thanks to Sam for this helpful URL: http://www.audiocoding.com/ Here they have a Winamp plug-in which is great, however you currently have to rename all your files from m4a file to mp4, which is a serious pain! (click --MORE-- for renaming help)
If you want to rename all your m4a files to mp4 to use, and need a quick way to do so, you have to drop down to DOS:
type CD
then type "ren *.m4a *.mp4"
No longer do you have to sit in Explorer renaming all your files one by one.