Free Anti virus programs

These are the ones I have used.  All of them are scanners, upon command, or perhaps automatically, they compare files on your disc with a list of known badware/malware, and post big "Gotcha" messages when they find a match.  They all take an hour or so to do your hard drive, and they all wait for you to personally OK zapping a malware file.  Just in case they make a mistake.  Which they never do.  Since new malware is hatched every day, they all have provisions to download updated badware lists.
   Most of them also install "realtime scanners" , programs that load into RAM at boot time and check all incoming stuff (email, email attachments, downloads) for badware,  These scanners slow your system down a lot.  I just removed a couple from my desktop, and the improvement in response is very noticeable and very pleasant.  MicroSoft Security Essentials is the worse offender here.  It slowed Blackbox down so much he was falling behind my typing.  When a 600 Mhz machine with a Gigabyte of RAM cannot keep up with my 10 keystrokes/sec hand typing, something ain't right. Blowing away Microsoft Security essentials fixed things right up.
   I have run all of these antivirus programs within the last few months.  The computers all survived the experience.  They are all free.  They are all fairly easy to run, you download them from the web, run 'em,  and tell 'em to zap everything they find.  None of 'em will find everything.  If faced with a difficult infection, try several of them.  What one misses, another might find.   

MalwareBytes.  A relative newcomer.  As a plus feature, no "realtime scanner".  He just scans the hard drive, once, and doesn't bother you afterwards.  Website BleepingComputer tipped me off to this one.

Spybot Search and Destroy  Been around for a while.  Started out as an anti spybot scanner and broadened out into scanning for most types of malware.  No "realtime scanner", a plus feature in my estimation.

Lavasoft AdAware  Another spybot scanner that has branched out.  Installs its own "realtime scanner" and whines when it finds traces of other antivirus programs on your machine. 

AVG   Another relative newcomer.  Still shaking the code down, but it works and does no harm.
 
ZoneAlarm.  Started off as a firewall and the latest version includes a disk scanner and a "realtime scanner".

MicroSoft Security Essentials   Fairly new from Redmond.  Effective, is able to block some pesky infections spread from websites.  CPU hog.  Slows your machine a LOT, all the time. Consumes up to 90% of your CPU time.  Difficult to turn off or remove.