I keep trying to install the new itunes update 11.0.1 and I keep getting the message that it has an invalid signiture and wont install. What can I do to update this? I am running windows XP.
That suggests that the installer is getting damaged during the download.
I'd first try downloading an installer from the Apple website using a different web browser:
If you use Firefox instead of IE for the download (or vice versa), do you get a working installer?