Fourth-place NBC, suffering one of its worst springs in the network's history, fired chief entertainment executive Kevin Reilly Tuesday only three months after giving him a new three-year contract. Reilly was replaced by the two-man team of Ben Silverman and Marc Graboff. Silverman owned a production company responsible for bringing shows like 'The Office' and 'Ugly Betty' to American television, while Graboff has been a veteran TV executive concentrating primarily on the business side. Despite developing 'Heroes' and receiving critical praise for shows like "Friday Night Lights," Reilly had been unable to stop NBC's long slide from the heyday of "Friends" and "Seinfeld."