1. Check the option in the Appearance tab of settings.
or
2. Repair your avast installation through Control Panel.
or
3. Make a link to ashdisp.exe in your startup folder
or
4. Add the path to ashDisp.exe into a value named avast! in the Windows Registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
See picture here:
http://forum.avast.com/index.php?topic=26155.msg213891#msg213891If that does not help, or you don't want to try, please, uninstall, boot, install again, boot.