How can I I'd the invalid signature when trying to upgrade iTunes on my PC?
Have you tried to do a repair install on the pc of iTunes? Also if that does not work i would follow these instructions on removing iTunes and reinstall. http://support.apple.com/kb/HT1923 for windows vista and 7 and http://support.apple.com/kb/HT1925 for windows xp. let me know if that helps.
Retrieving data ...