Error 5 means access denied. Sorry, that’s the info…
Did you boot after installing?

Do you use XP or Vista?
Do you use any other antivirus in your computer?

Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.

If this does not help, can you uninstall / boot / install / boot again?