One of the most unintrusive antivirus programs I have found is AVG. I have installed it on many of my clients computers and it has performed very well. It doesn't seem to slow down the computers at all and keeps them fairly safe from viruses. Please check it out.
Computer and internet use is an integral part of our everyday lives. Every aspect of our lives is accessible electronically, which brings considerable benefit – and, unfortunately, considerable risk. Our role is to shield you from that risk by providing powerful, easy-to-use protection that works efficiently and invisibly in the background so you can focus on using your computer, not worrying about security.
As the nature of threats continues to evolve - from computer- and web-borne viruses to complex exploits designed to steal information for financial gain, the nature of protection must also evolve. Using the latest technology developed by world-renowned security experts, AVG provides real-time defenses against real-time threats. With AVG software on your system, you can safely surf the net, create online profiles, shop, bank online, download files, store data, chat, and email, safe in the knowledge that your personal and professional information will not be stolen or misused.