Cannot update myApple software or iTunes 10.7

Trying to update my apple software and iTunes and during install it came up with error message that says "Errors occurred while installing the updates. If problem persists choose Tools> Download Only and try installing manually. When you try to install manually I get an error that says “iTunes has an invalid signature. It will not be installed”.

What is wrong as I have never had an issue updating software and/or iTunes before.

Any ideas about this invalid signature error?