That's strange I had that on one occasion with 7.0.1465 RC build, but I have been doing manual program updates hoping to pick up the new regular release version 7.0.1466. But even though for some reason it isn't available on my RC build, it hasn't asked for a reboot as it did once before.
Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.