First of all, do you have another anti virus installed? If you do, one of them has to go.
To remove Avast, first, hold the hammer, no matter how much your Avast looks like a nail. Download the removal tool from the site, turn off Avast's self-protection module, run the tool (be careful while using it, you could wipe your hard drive clean with this thing, so make sure the right folder is specified in the bar), reboot, then download the program from the site and do a clean install. See if that works.