How do you fix a virus if your computer won't start?

I recently downloaded a virus on my computer by accident, as soon as the download was done my computer immediately went to a blue screen. My problem is that whenever i try to start my computer a black screen comes up giving me the option to start my computer normally or in safe mode. I tried starting it both ways and it goes to a blue screen then powers down, starting the whole process over. Does anyone know how fix this?