It is actually incredible how much very good antivirus software is available for free. Even Microsoft has a package out there that does a pretty good job of protecting Windows machines from viruses and it is available for free.
One thing to keep in mind is that finding a software package that fights viruses as well as adware/malware can be difficult. A lot of people have an antivirus program running as well as other scanners that stay on the lookout for adware/malware.
By the way, this isn't as much of an issue with Linux. Some people say that people don't target Linux with viruses because so few people use the operating system, while others claim people don't target Linux (or Mac OS, for that matter) because so few people use it and a lot more damage can be done by targeting Windows machines.
I'm not sure what the truth is. I do know I wouldn't venture onto the Internet without a good virus scanner in place and I do have one that looks at adware/malware, too.