Hello Sad Gamer,
The top 3 Free antivirus programs are usually, Avast, AVG and Panda, in not particular order.
It all comes down to which one you prefer, however I personally have a slight preference towards Panda, because the impact on system resources is fairly low.
Here you can download it : http://www.pandasecurity.com/usa/homeusers/solutions/free-antivirus/
Best of luck,