Nearly all of them can be downloaded free for a 20 or 30 day trial. A few have some features disabled. Some are actually freeware, written by people for their own use originally, I am told some of these are as good as the bought ones.
Watch out for sites that say "free download" - only means they charge nothing forthe download, not that software is free. In some cases the download is free only after you pay to become a member.
Also I would keep away from the free on-line scan ones, I would not want them looking inside my machine. If on-line scan antivirus ones are anything to go by, all they do anyway is SCAN, then tell you 100 problems and charge you to fix them.
Whatever OS you have, do a system restore point before trying any of these.