System Restore can have unforeseen circumstances and avast (self-defence module) doesn't like what it tries to do with avast. This can cause corruption unless the self-defence is disabled before using SR.
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.