All the OP's are using Win7 or Vista. The solution was posted by shacl, and is a common problem we see here. Many new machines come pre-installed with a trial version of an antivirus on them. Then the user installs Avast and it doesn't work. This is because they first need to uninstall the trial AV because you cannot run 2 AV's at the same time or they will conflict and cause chaos with your machine.
So when you buy a machine, find out what AV is pre-installed, uninstall it using the vendor's uninstaller tool (NOT a third-party uninstaller), then reboot. Then you can download and install Avast.