A week dominated by Fox's airing of the Super Bowl also resulted in a historic low for NBC (search).

NBC dropped to fourth place this season among viewers aged 18-to-49, the first time it has ever been so low this late in a TV season. That's the demographic NBC most cares about because it's the audience the bulk of TV advertisers pay to reach.

NBC has won among this group for four years in a row, and eight of the past nine years, and as a result earns more than its rivals every spring when advertisers lock in commercial space for the upcoming season.

It's very close, but so far this season CBS leads among these young viewers, and has a commanding lead among viewers of all ages. ABC is second, and Fox moved past NBC this week on the strength of the Super Bowl (search). Among all viewers, NBC is a close third to ABC.

"It's parity among the networks now and that's what you're seeing playing out," NBC spokeswoman Rebecca Marks said.

She noted that NBC has a strong new hit in "Medium" and is optimistic about such future series as "Law & Order: Trial By Jury" (search) and "The Office."

Without an unexpected hit, NBC could have trouble climbing out of fourth. CBS has the most dominant day-to-day schedule, ABC has the Academy Awards and buzzed-about hits like "Desperate Housewives," and Fox has four months worth of "American Idol" episodes to run.

Besides the football game, last week was a good week for the Fonz and friends: ABC's 30th anniversary special for "Happy Days" was seen by nearly 20 million people. Fox is also seeing some promising numbers for the medical drama "House," seen by more people last week than the more-heralded "24."

Fox averaged 28.8 million viewers for the week (14.9 rating, 23 share), its numbers skewed by the Super Bowl. CBS had 10.5 million (7.0, 11), NBC and ABC both averaged 9.2 million viewers (NBC 6.2, 10, ABC 6.0, 10), the WB had 3.4 million (2.3, 4), UPN 2.9 million (2.0, 3) and Pax TV 680,000 (0.5, 1).

NBC's "Nightly News" won the evening news ratings race, averaging 10.4 million viewers (7.3, 14). ABC's "World News Tonight" had 10 million viewers (6.9, 13) and the "CBS Evening News" had 7.5 million (5.3, 10).

A ratings point represents 1,096,000 households, or 1 percent of the nation's estimated 109.6 million TV homes. The share is the percentage of in-use televisions tuned to a given show.

For the week of Jan. 31-Feb. 6, the top 10 shows, their networks and viewerships: "Super Bowl XXXIX," Fox, 86.1 million; "Super Bowl Postgame," Fox, 50.1 million; "American Idol" (Tuesday), Fox, 28.5 million; "American Idol" (Wednesday), Fox, 26.2 million; "CSI: Crime Scene Investigation," CBS, 25 million; "The Simpsons," Fox, 23.1 million; "Happy Days 30th Anniversary Reunion," ABC, 19.9 million; "Without a Trace," CBS, 19.3 million; "ER," NBC, 18.1 million; "Everybody Loves Raymond," CBS, 17.6 million.