The lead detective on the hit CBS series “CSI: NY,” Mac Taylor, is a pretty conventional television hero: like his colleagues on the two other “CSI” franchises, he uses science to follow the evidence and catch the bad guys. But in the episode for Oct. 24, Taylor, played by Gary Sinise, finds himself entering the computer-based virtual world known as Second Life, walking (and sometimes flying) around three-dimensional, animated Manhattan landmarks, recreated by a technique called machinima, and pursuing not a suspect but an avatar.