I've been using Spybot S&D for over 4 years. It used to find "tracking cookies" and similar stuff regularly until I set Firefox to refuse cookies from the offending domains. Now it rarely turns up anything new. By the way, it's full name is Spybot Search and Destroy. It's not a spybot -- it searches for spybots and destroys them.

Anti-spyware programs and anti-virus programs do different things so I've been using one of each. I've known people who find themselves at high risk for viruses and/or spyware to update and scan daily. I have an older system that seems to be a less attractive target, I do relatively few downloads, and I don't seem to visit many virus-infested sites, so I've been getting by OK with updating and scanning about once a week.

Does Spybot S&D have a paid version? It's been free for as long as I've known it. Safer Networking, the developer, has said they welcome donations and I've sent them a few bucks to help them keep doing their thing. I used to pay AVG (an anti-virus) for a two-year license but I'm going to let mine expire until I again have a system that they support. Avast! (another anti-virus, the one DocJohn recommends in this thread) offers a choice of free and paid versions. I haven't found any need for the paid one, especially considering how I have to keep the free one on a short leash to keep it from interfering too much with my system.