I hope this helps, as I have been having no end of problems too (firstly iTunes crashed my wireless router, then wouldn't launch after XP restore, then wouldn't uninstall, couldn't reinstall...etc) Realised it is pointless trying to get 5.0 to work AT ALL so the best thing to do is get 4.9 back -- of course though, that seemed impossible too! Here's exactly what I did
1) Tried to unistall iTunes using add/remove programs, which didn't get rid of it but I assume got rid of some of it.
2)Uninstalled quicktime using add/remove programs.
3)Downloaded and installed the new STAND ALONE quicktime from
http://www.apple.com/quicktime/download/standalone.html
4)Downloaded and installed 4.9 from
http://www.5star-shareware.com/Windows/Music/MultimediaPlayers/itunes.html
5)At this point, iTunes installs but won't run because some file or other (not 100% which one) was created by "a newer version of iTunes". I then manually deleted the only files I could find that might be interfering, namely the "Library" text file and the iTunes folder in My Music.
iTunes would now launch and work fine but of course had no music in it. Clicking "Add folder to library" for My Music worked fine for me and created a new 4.9 library-yay!
All my music returned and worked (MP3s and Protected AACs) and I shall make a studious point of NOT downloading 5.0 until a version that works is released.
One thing; I would back up all music to a seperate location on the Hard Disk before doing this as I guess deleting the iTunes folder in My Music could delete some people's music altogether. It didn't for me but I don't want to be responsible for someone losing half a million songs or something! You can always add the music files back in after deleting the library.
I'm pleased as punch to have my nice ol' 4.9 back and really hope this works for everyone else who just wants to get theirs back too.