If you are getting a changed file alert, the comment given in a couple of computer forums is that the file could have been updated during a Windows Update. The change can be accepted by opening the AVG Test Centre, pressing the F3 key on your keyboard and opting to accept the changes. It would be a different matter if the message said that the file is infected.
It's a bit of a unique message if it says, 'A weird thing has happened to shell32.dll' (just joking
). Seriously though, it is helpful to say what the exact message is when stating the problem.