Hi,
What is the operating system, is any other security software installed?
What happens when you try a repair via the control panel>manage programs?
If a repair can not work, I suggest a full reinstall from scratch.
-Download the full setup file, ~40Mb, from
here, and save it to your desktop. (do not download the universal installation file.)
-Download the Avast Uninstall Utility from
here.Go offline, if you aren't able to disable the Avast self protection module and uninstall Avast, reboot into safe mode,
-Run the file aswClear.exe. (The uninstall utility.) Reboot into normal.
-Install Avast from the setup file you downloaded.
-Reconnect. Get updates, All should be good.