May i know the browser which you are using on your PC, if you are using Internet explorer please first check the version of IE
Open Internet explorer - Help - About Internet explorer
If you find the IE version 7 please upgrade the version to IE 8, IE can be downloaded from Microsoft website (www.microsoft.com) free of cost.
And also perform these steps :
Click start > start search > type CMD > Run CMD as administrator by right clicking on CMD
On command prompt (black window) type the following commands and press enter on your keyboard
After which please try downloading McAfee software & check what happens, please feel free to reply back if any assistance required