It sounds as though your Avast has tried to update itself to version 5, and the update has not gone well.
There are a few reports of this happening on the forum.
A solution is often to go offline (because you will have no active AV while re-installing) and uninstall the software via the control panel, run one (or both) of the
Uninstall utilities (run both if you are not sure what version you had installed), reboot, and install the program afresh. Either 4.8, if you have the full installer file (can be obtained from the right-hand column
here -look for version 4.8.1368) or the latest version, from the same page/different download, or from
Avast. Two or more reboots after installation might be required in some cases.
I don't know why; that is just the reported experience of some users.
Naturally, you would be advised to get the installation/cleaning files needed before going offline and reinstalling the software.
Me, I'm happy with 4.8, for now, and have set program updates to manual. I want to see any bugs ironed out before updating. There were many bugs in the version 5 releases prior to the current.
It looks like 5.0.418 might be pretty bug free, a lot of success reported.