For a problem free online protection, this is one the best antivirus software with "Resident Shield" (online protection) you may find. This feature welcomes trojans and viruses when they attempt to leak into the system and see them off. So you don't have to spend your valuable time for cleaning the system. You can also enjoy daily virus definition updates for free. Just give a try.

Avast Antivirus Home Edition Features
Antivirus kernel, Automatic updates
Simple User Interface, Virus Chest
Resident protection, System integration
P2P and IM Shields, Integrated Virus Cleaner
Network Shield, Support for 64-bit Windows
Web Shield, Internationalization.