Please search the forum for similar topics. Older security tools, not running the appropriate removal utilities (http://thewebatom.net/uninstallers/security-software/), and optional connected hardware are some of the reasons.

If you used to had some other security tool, search for the removal utility and run it under Windows Safe Mode.

BTW, you shouldn’t run a restore point for this type of things. It tends to complicate the situation instead. If you can get to safe mode, then running the Avast removal tool http://www.avast.com/uninstall-utility directly from Safe Mode (skipping the “Add/Remove Programs” step) should solve most problems.

After installing Avast under Normal Mode, reboot before anything else.

Unplug / disconnect / disable any optional device (wifi, modems, usb devices…).

If all these fail, please don’t hesitate to post again with details of OS version, edition, x32 / x64, SPs… and Avast version and edition.