"Your system has not been modified" error

I've tried to update to iTunes 11.4 on my 64-bit Windows 7 laptop and it didn't work out, so I decided to uninstall then re-install it. Still didn't work, I also kept getting "access denied" messages after I opened the iTunes file to install it. This is really frustrating for me since I know I have all the requirements to be able to download it. I've tried many other solutions as well and none of them are working.