What is the best antivirus program to use that is affordable

[Jondar]

i have an Hp Pavillion a6430w,and today i had to restart it since i had installed a free trial of an antivirus,anywyas it would not restart it would get to the part when it loads on the bottom middle of the screen, but it just kept loading and not going anywhere, i had to restart it several times for it to finally work. any one else have this problem or know what might be causing this? my PC has shitty Windows Vista unlock iphone 3g