Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
Have you rebooted - if not try that.
If that didn't work - 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.