On a comnputer running Windows, about how often should one find a virus on it?

[Razorflame (US)]

My computer runs Windows, and I use a great antivirus program, however, about once in a couple of weeks, it finds a virus on my computer. I do visit a lot of different websites, however, this doesn't account for the viruses showing up on my computer. About how often should one find a virus on their computers?

Thanks for the help in advance!