Dimitar Berbatov's angle finish put them ahead after 22 minutes, with Albion Paul Robinson sent off before half-time for fouling Ji-sung Park.
Carlos Tevez scored seconds before the interval after Scott Carson dropped Ryan Giggs free-kick, and Nemanja Vidic headed home a third on the hour.
Cristiano Ronaldo then struck twice in eight minutes to seal United's win.
It gave Sir Alex Ferguson's side a three points lead ahead of Liverpool in the title race, with the added bonus of topping up their goal difference in style at The Hawthorns.
United also made it a record 11 straight clean sheets in the league as the pressure piled up over Liverpool and Chelsea, who are in action in Wednesday.
Edwin Van der Sar have surpassed Chelsea's Petr Cech, breaking his record of 1,025 minutes without conceding a goal.
Rio Ferdinand came back after a back injury and got a good game. Zoran Tosic made his debut with The Red Devils, after joining 1 month ago.
It was a comfortable night for United, but not for the club's hoping to take away the Premier League crown.
United Line-up: Van der Sar, Neville (Eckersley 71), Vidic, Ferdinand (Brown 70), O'Shea, Carrick, Giggs, Ronaldo, Berbatov (Tosic 77), Park, Tevez.
Subs not used: Kuszczak, Scholes, Fletcher, Gibson.
Booked: Park and Carrick.
Goals: Berbatov 22, Tevez 44, Vidic 60, Ronaldo, 65, 73.
Referee: Rob Styles.
Man-of-the-Match: Ryan Giggs.
Source: BBC Sports.
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