I need help installing AVG 8.0!?!?

Did you restart your system at all, after getting the error message? It could be like it says in the error that, you'd un-installed ur previous version of AVG, but haven't restarted your system since.. Try restarting.. Even if that doesn't work, you might need to download some sort of a registry cleaner (read reviews before you download anything, some might be spywares/viruses) and remove some values from ur registry..