It would have been better to have uninstalled AVG 7.5 completely, then rebooting the computer before installing AVG 8.

You can try a System Restore to a date/time before you first attempted to install AVG 8, but I am not sure this will work as it seems the normal Add/Remove Programs method was not carried out by the system properly.

Goto Start > All programs > Accessories > System Tools > System Restore, select "Restore my computer to an earlier time", click Next, select a date/time before you started this upgrade, and then follow the prompts.

If that works, I would test whether AVG is still working properly before attempting the upgrade again. I would go to Grisoft's web site and download the setup files from there, but make sure AVG 7.5 is fully uninstalled before installing AVG 8.

I hope that helps.