There are many free programs as well as many virus's pretending to be anti-virus.
Here is a list of the top, free, legal, safe anti virus programs.
AVG (free) 10
Avira personal edition
Microsofts Security essentials: It is supposed to be a good produt, but if you read the acceptance part, it says they will look at your hard drive and send whatever info they want about YOU to M$, without asking.(If thats ok with you, feel free to download and use it.) Also, M$ just introduced a "small business version" covering several servers and up to between 50-79 client computers.
Also available, but not the best is:
PC Tools antivirus
Rising anti virus.
No, this is not all of them, just a taste of the best free ones (no trial's)