Cook threw for two touchdowns. Langford ran for two scores, and the Spartans (10-1, 7-0) reached the conference title game for the second time in three years.
Langford ran 20 yards untouched for the game's first touchdown in the second quarter. He sealed it with a 37-yard scoring run in the fourth after Kurtis Drummond picked off a short pass by Trevor Siemian, sending Michigan State to its seventh straight win and Northwestern (4-7, 0-7) to its seventh loss in a row.
Cook completed 16 of 24 passes, including an 87-yard TD to Bennie Fowler on a one-handed grab in the second quarter and a 15-yard scoring strike to Josiah Price in the third.
Drummond had two interceptions, and the Spartans closed in on 11 wins for the third time in four seasons. They're also eyeing their first trip to the Rose Bowl in 26 years.
Safety Isaiah Lewis knocked Northwestern's Kain Colter out of the game with a big hit as he tried to catch a pass early in the first quarter, resulting in a targeting penalty and an ejection.
Things didn't get much better for the Wildcats after that. They fell out of bowl contention after making one the previous five years and could go winless in Big Ten play if they don't win at Illinois next week.
With Colter out, Trevor Siemian carried the load at quarterback and was 25 of 43 with two interceptions.
The Wildcats managed just 80 yards rushing, and their freefall continued after a 4-0 start.
The tailspin began when they blew a 10-point in a 10-point loss to Ohio State, and they haven't been the same since. They got blown out at Wisconsin and suffered a string of gut-wrenching losses, whether they were losing on a last-second heave in regulation to Nebraska or falling in triple overtime to Michigan last week.
Things took a bad turn for them in this one when Colter got drilled by Lewis, resulting in the ejection.
Colter stayed down for a few moments and walked off clutching his midsection. He did not return.
Jeff Budzien kicked a 22-yard field goal on that drive to give the Wildcats a 3-0 lead, but Michigan State went ahead midway through the second when Langford ran 20 yards untouched to the end zone.
Fowler added to the lead on Michigan State's next possession when he reached back for a one-handed grab along the left sideline as Northwestern cornerback Matthew Harris slipped. The result was an 87-yard touchdown that made it 14-3 with 4:31 left in the half.
Northwestern then drove from its 28 to the 3 but was forced to settle another field goal by Budzien, cutting the deficit to eight with just under two minutes remaining in the second quarter to make it an eight-point game.
Michigan State's Michael Geiger kicked a 37-yard field goal on the opening drive of the second half after a wide open Tony Lippett caught a 48-yard pass from Cook on the first play.
The Spartans added to their lead when Cook fired a 15-yard bullet over the middle to Price that made it 23-6 with five minutes left in the third. Chance Carter blocked the extra point and Northwestern was moving the ball on the next possession when Darqueze Dennard picked off a deep pass along the left sideline.