I personally use Avast and I like it. Avast is offered for free for home use only. It updates like the more popular virus programs and in my opinion is just as effective. What it doesn’t do is schedule when it will automatically scan your computer so you have to start the program yourself. They offer the professional version (which is not free) which can be set for automatic scanning.You should not run more than one virus protection program as they will conflict with each other.
The following programs are also FREE. Whether you pay or decide to use one of these programs, you need a spyware program just as much as an anti-virus program. In some cases spyware can be worse than a computer virus as it can be used to steal your identity, or even take control and use your computer for illegal purposes. I use Ad-Aware and it seems to offer everything I need to clear out spyware and adware.