Need for free antivirus

my computer is at risk, wat should i do? i dont have enough money to purchase any anti virus...so i joined here so that if by chances anybody and sombody can help me.
im confused and going crazy coz some one in my past computer have risen to my present computer, i remember those days dat give me smiles and heart pumping excitements but then my past was just an awful as those people describes.... der was a time dat i cannot used my computer coz it was totally black out, im not a IT grad so i just leaved it dat way..
well its so much to ask but does anyone knows what is my problem really is? i cant decide?