In order to do that with no other external hardware (like a Roku) your PC would have to have a video card with a TV output (if you have a standard-def TV), or if you have an HDTV it may have a PC input you can hook the video cable up to.
Usually newer tvs, or computers have either HDMI or VGA slots, just plug the cable from the pc to the tv if they both have vga or hdmi, if it only have component slots than youd have to purchase a vga to rca cable heres an example:
http://www.amazon.com/VGA-RCA-Component-RGB-Cable/dp/B000FM3EQ0
when hooked up just go to www.netflix.com and log in and you can instant stream