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Its absolutely impossible to function on the internet these days without practicing safe computing (and I'm not talking about wearing a write-protect tab). You need a good set of virus protection, and optimally a decent spyware protection unit as well, or else your computer will slow to a screeching halt as spammers and DDOS operators use your system as a platform for their unsavory business.
My personal favorite anti-virus software, ALWIL's Avast!, celebrates its 40 millionth user today on its Home Edition. It's particularly impressive as its 20 millionth user was registered but a year ago in October of 2006. Avast has a fairly long history of protecting computers, having been around since 2002.
I've been using a combination of my favorite spyware scanner and Avast! for a few years without significant infestation. Download.com approves as well, giving it their 5-Star Rating. Home Edition is available free of charge (my personal favorite price), and downloadable in 30 languages.