I recommend none of the free Antivirus solutions. Avast currently seems to be the least bad one. The commercial solutions offer better protecion than the free ones. All virus scanners quite frequently create crashes of the system. They also slow down performance. There is no 100% secure protection.
This really helps:
- Updating your OS regularly is the most important point
- Stay away from illegal places in the internet
- Disconnect devices from the internet, which don't have to be online
- Regularly create backups of your data
- Do not click on attachments in emails
- Uninstall Adobe Reader and Adobe Flash
