Reports on the Apple support forums this morning ( http://discussions.apple.com/thread.jspa?threadID=2092831&start=0&tstart=0) are that users are no longer able to able access i-Tunes.  It seems that an AVG anti-virus update overnight is reporting that i-Tunes files are part of a “small.BOG” virus/Trojan.  This is causing the AVG program to quarantine files and if you say yes to delete when you are warned about your machine being infected, causing i-Tunes files to be deleted.  
    A work around posted on the apple forum ( http://discussions.apple.com/message.jspa?messageID=9877057#9877057 )   is to add "C:\Program Files (x86)\ipod" as an exception under Resident Shield>Manage Exceptions>Add Path. For those who still have the update from AVG that cause the problem.  

     Additionally, there are reports that AVG has fixed this issue with an update this morning, so you do want to make sure your AVG software is updated. Depending on if AVG only blocked the i-Tunes files or actually deleted them, will then tell you if you need to just restore them from the AVG program or reinstall i-Tunes.

     This is only that latest false positive problem that has appeared for anti-virus software makers in the past few weeks, recently CA (  http://techgeekandmore.wordpress.com/category/ca-anti-virus/ ) and McAfee ( http://techgeekandmore.wordpress.com/2009/07/04/alert-if-your-a-mcafee-anti-virus-user-do-not-update-your-av-until-you-see-this/ ) have both seen that same type of issues.