Now you are running with standard shield on Normal, does this have a reduced effect?
When it was running at 100% was the avast icon rotating, indicating it was scanning something?
If so, open the on-access detailed view for Standard Shield and see what file it is scanning.
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 try, uninstall, reboot, install, reboot. It would probably be a good idea to download the latest version to use when you have uninstalled.