Have you tried the stupidly well hidden Avast repair feature?

With Win XP it’s: Control Panel > Add Remove Programs. Highlight Avast and click the Change/Remove button to find a program/settings menu including a Repair option.

This has worked for me on at least two occasions restoring Avast to working state including once after a system restore had messed it up in some way.

‘Simply’ uninstalling/reinstalling Avast may work but I know from my own experience that can cause other issues because it doesn’t remove every element of the previous installation. A full uninstall requires a special software tool Avast provide for that purpose.