Windows 7 - Itunes won't install since there is no digital signature

I just got an Iphone 4 and therefore needed to upgrade my Itunes to Itunes 9. When I tried to upgrade through Itunes itself the process just stopped midway. So I uninstalled Itunes and Quicktime and deleted all related files, also the temp files. When I tried to reinstall itunes 9 my computer blocks it due to invalid digital signature. I have tried downloading it using both firefox and internet explorer, I have also tried to unblock it by right-clicking and going into properties. It just won't start up! Oh and I am using windows 7 professional.

Would be greatful for any tips on how to solve this issue as I would like to get started using my iphone!