The following youtube videos will explain how to set up AVG Anti-Virus Free Edition v8.0.1
The first one deals with installing AVG V8, there is no need to uninstall V7.5 as this will be done automatically during the setup. It is important that you close all running programs and any open windows.
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AVG AntiVirus V8 Problems
AVG Anti-Virus Free Edition V8.0.1