Install again from scratch:
1. Download avast internet security from
here. (If you already have the latest, then no need)
2. Download avast uninstall utility from
here.(Disconnect from the internet after downloading the required files.)
3. Uninstall avast from add/remove programs.
4. Start Windows in Safe Mode
5. Open (execute) the uninstall utility
6. If you installed avast! in a different folder than the default, browse for it. (Note: Be careful! The content of any folder you choose will be deleted!)
7. Click REMOVE
8. Restart your computer into normal mode.
9. Install avast IS from the setup you have downloaded in step 1.
Do not install the license key yet. Register for a trial version. If that works, install your genuine license key.
Let us know what happened.