mouse doesn't work on the a icon

Hi. I have a weird problem.

I’m trying to change the setting but the no reaction with clicking the a icon on the system tray.

any idea?

Do you mean the mouse doesn’t work or the avast a icon doesn’t work? I see the title you wrote is mouse doesn’t work on the a icon but you said

I’m trying to change the setting but the no reaction with clicking the a icon on the system tray.

any idea?

Which is your Windows? 98, Me, XP, Vista?
Do you use any other antivirus in your computer?

Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair).
If this does not help, can you uninstall / boot / install / boot again?