I have a g4 with osX and all of a sudden itunes won't open. Safari works fine, sometimes quicktime won't open, but itunes just sits there trying. What is wronG!!!
which version of OS X do you have? which version of iTunes and quicktime? when did this start to occur?
have you tried to repair permissions on your hard drive?
Repair permissions: Open Disk Utility (Hard Drive --> Applications --> Utilities --> Disk Utility) and click on "Repair Permissions"
Remove itunes.plist: Go to Hard Drive --> Users --> your user name --> Library --> Preferences --> com.apple.iTunes.plist and drag that file to the trash.
Note: if you see com.apple.iTunes.eq.plist or com.apple.iTunesHelper.plist you can throw those away, too. (though they most likely aren't the problem)
the .plist files associated with every application contain all the preferences for that program since you installed it. binning wipes the slate. when the program opens, it'll look for it's .plist, if it can't find one, it creates a new clean one.
create a new user in accounts in the system preferences. make it with admin capabilities. log out of your current user and log in in the new one. does itunes open?
It occurs that your library could be corrupt. Try Lt Major Burns' suggestion first. If it works, then log back into your own account, and throw away your home folder --> Music --> iTunes --> iTunes Library & iTunes Music Library.xml
Don't worry, you'll be able to rebuild the library once you get iTunes to open.
If all this fails, iTunes could simply be broken. Try downloading it again from http://www.apple.com/itunes/download/ and reinstalling. You may have to search for everything with "iTunes" in the name and throw it away first (except for your "iTunes Music" folder!)