Operating system restart needed by avast message - First check the contents of the C:\Program Files\Alwil Software\Avast4\Setup\setup.log file, which should show you why it thinks the reboot is needed.
This may be solved, in some systems, by deleting the file C:\Program Files\Alwil Software\Avast4\Setup\reboot.txt. With the avast self-defence module it will complain so disable that first, Program Settings, Troubleshooting.