If you want something that's freeware, relatively lightweight and for Windows and for home and small office use, then you can get Grisoft AVG Free Edition, which actually works fine, but has less options than what you have to pay for. Even virus definition updates are free (I don't know for how long, but I have had these programs installed for long enough). The major version offered is a figure lesser than what the site (
http://www.grisoft.com ) offers for pay.