While originally known purely as a tool to combat and remove "adware", Ad-Aware has matured to the point where it is just as good as many antivirus programs. The software will scan your computer relatively quickly, and you will often be surprised at how much malware it detects. Another plus about Ad-Aware is that you can choose to not have it run in the back-ground. This prevents it from using up your PC's resources if you are doing something such as playing a game.
These are two free alternatives that can be compared to paid products like McAfee. AVG is slightly faster, but Avast may be more thorough. Avast would be a good option if you have an at-risk, or severely infected PC. If you use Avast, be sure to let it run overnight so it can do a full scan. Both AVG and Avast also have paid options that give you more features and better support.
This is one of the few non-corporate pieces of virus scan software. Similar to Ad-Aware, ClamWin can be disabled so it does not use up your computer's resources. The fact that it is "open source" means that there will probably always be lots of people out there providing support and updates. This also means that you will not be tied to one company and get surprise charges/etc. If you use ClamWin and like it, you are encouraged to donate whatever you can to the project on their website.
Microsoft Security Essentials
According to its website, Security Essentials touts itself as the "anti-annoying, anti-expensive anti-virus program." At first glance of the home page, it definitely provides a clean, no-nonsense experience. Because it is made by Microsoft, Security Essentials should theoretically run much smoother on your Windows system. Also, because Microsoft is a well established company, they do not need to rely on annoying banner ads or paid updates for their virus scan software. This can be a huge difference from other virus scan software. Finally, because Security Essentials is free if you have a genuine copy of Windows XP/7/Vista, the price should not be an issue.
Switch Operating Systems
The best way to get rid of your virus problem is to switch to a different operating system. The "operating system" is what you computer boots into and uses to run other programs and manage the internal hardware. The reason Windows gets so many viruses is because most of the world uses it. Think about it: if you were writing a virus, you would want it to target the most people, right? Currently most people use Windows, so naturally most viruses are designed to target Windows. If you are really sick of viruses, consider picking up a Mac, or if you're brave try installing Linux.