Operating system restart needed by avast message could be solved, in some systems, by deleting the file <avast>\setup\reboot.txt
If deleting reboot.txt doesn't stop the requests for reboot check this:
Check this registry entry (right after computer restart):
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager, if it contains "PendingFileRenameOperations".
Check its contents and if there's only "reboot.txt" from avast4\setup folder, delete PendingFileRenameOperations, don't delete any other key.
Back-up (export) the registry key before you edit/delete, just in case.
Merry Christmas!