Q: Itunes wont install on vista, No digital signature

I had a problem were Itunes would not update or anything Apple related (quicktime etc) so i deleted Itunes and tried to run it as a fresh copy.
Yet when i download it and try to run the application i get the box from windows asking me if i want to run it, but in this box next to publisher it says unknown.
When i click run nothing happens. The box disappears and thats that.

If i click on the application and go into properties and go into digital signatures it says the digital signture is not valid. Ive tried downloading it with Explorer, Firefox incase that was it. I have even done a clean install of windows and tried to install itunes first incase there was a conflict.

Can anyone please help because its driving me nuts!