I suggest an installation from the scratch:
1. Download the latest version of
avast! Uninstall Utility and save it.
2. Download the latest
avast! version and save it.
3. Uninstall avast from Control Panel (if possible). If, for any reason, you can't run it, try booting in
Safe Mode and doing it from there. Anyway, boot after that.
4. Run the avast! Uninstall Utility saved on 1. If, for any reason, you can't run it, try booting in
Safe Mode and doing it from there. Anyway, boot after you've run it.
5. Install avast! using the setup saved on 2. It will be good to accept the boot time scanning. Boot.
6. Check and post the results.
If that does not help, well, most probably you're infected and it's preventing avast to install and load correctly.