So yeah, no spending on anti-virus. I want something which is not annoying and easy to uninstall and effective.
Open-Source - dunno about that. But for Windows you can safely rely on Microsoft Security Essentials. It comes free (separate download) and gets updated frequently.
Well, first line of safety is always the user. Generally this + Windows Firewall - provide ample protection. Its not flashy but gets the job done, Just keep it updated.