I'm a fan of AVG Free. I have never had any problems with it.
Many people ask us for free alternatives to subscription based anti-virus and anti-spyware programs. For anti-virus I use avast [avast.com]. It's free for personal use and I have been running it on more than 10 machines for years now, without issue. The only thing I recommend is that once it's installed, go through and setup the preferences to remove audio alerts and other things like that. Also, make sure to let it scan the entire hard drive when you first install. You'll be surprised how many viruses have appeared on even a relatively new machine in a pretty short amount of time.
For anti-spyware I recommend ad-aware. They have a great free version and you can download it atlavasoft.com.
Another option if you don't want to download something right away is the free anti-virus scan provided by Trend Micro. You can access it at housecall.trendmicro.com.
IMO, AVG (free.avg.com/) works well enough, but it can be a bit buggy. For example it hasn't been updating properly lately so I have to right click the icon in the toolbar and tell it to update manually. Also, it may be that my system is getting older, but it seems like scans last for ever! I have them set to run on Saturday afternoon, so I normally miss them, but they can crank on for over two hours. That said, I haven't had a virus problem as long as I've had it installed so I can't really complain.