i can't download the new update

Do you get an error message when you try to install iTunes? If so, what does it say? (Precise text, please.)

I get a message "invalid certificate"

That suggests the installer is getting damaged during the download.

I'd first try downloading an installer from the Apple website using a different web browser:

http://www.apple.com/quicktime/download/

If you use Firefox instead of IE for the download (or vice versa), do you get a working installer?