Microsoft Security Essentials.

I've had Norton, McAfee, and some others I can't remember the name of, but I'm not sure why I waited so long to load the MS a-v program, Security Essentials. It fits fine anywhere in your machine, it's made by MS (I'm assuming you have a MS system?) and it's FREE!!! Works totally behind the scenes. Doesn't constantly bother you about something or another like the others did. I haven't had any problems with the MS program in any way.

Here's link to the MS website & free download.
http://www.microsoft.com/en-us/security_essentials/default.aspx