Whilst this isn't a problem if you know avast is working, it is a pain in the rear. You can if you wish disable this check, but I'm sure you would rather have it working. It isn't easy to pin down why this happens so we will try to ensure that avast is totally 100%.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work, download the latest version of avast from the web site and try, uninstall, reboot, install latest download, reboot.