First try a System Restore to a point before you uninstalled 7.5 & installed 8.
Then shut down any running programs (incl 7.5's running modules in Task Manager), & try the install again. I installed 8 over 7.5, & it uninstalled the previous version in the process.
A few years ago (when I first began using AVG), I coudn't get the email scanner to work no matter what I did. AVG released a new version some months later & it started to work. Let's hope it's not one of those! G