An installation from the scratch:
1. Uninstall avast from Control Panel first.
2. Boot.
3. Download the latest version of
Avast Uninstall and use it for complete uninstallation. If, for any reason, you can't run it, try booting in
Safe Mode and doing it from there.
4. Boot.
5. Download, save and install the latest
avast! version. It will be good to accept the boot time scanning on next boot.
6. Boot.
7. Check and post the results.
should I turn off protection before uninstall
No.
will I have trouble if I try to use old product key after reinstalling?
No, the key is valid for 14 moths since it was generated.