United celebrate the opening goal
Manchester United produced a dominant second half display to see off Watford.
A Cristiano Ronaldo penalty midway through the first half put them ahead after Jay DeMerit had pulled back Ole Gunnar Solskjaer in the area.
Watford nearly equalised when Chris Powell's cross hit the post but when Lloyd Doyley deflected in Kieran Richardson's cross it was all but over.
Substitute Henrik Larsson sidefooted home his first Premiership goal and Wayne Rooney scored with a fine lob.
It was a routine victory for Sir Alex Ferguson's men, who made it nine straight wins at home and maintained their six point lead at the top of the league.
Watford went into the game 42 points behind United, without a win away from home and having never won a league game at Old Trafford.
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And with the Hornets struggling with injuries at the back it looked ominous, particularly when Rooney opened up the defence for Solskjaer in the opening minute only for Jordan Stewart to intervene.
But United had obviously been studying the formbook and with overconfidence flowing through the side they were guilty of some seriously sloppy play.
Rooney went close with a header from a Richardson cross but other than that United were finding it hard to break through Watford's defensive roadblock.
They were given a helping hand when DeMerit wasted all their early efforts by needlessly tugging on Solskjaer's shirt and Mike Dean had little hesitation in pointing to the spot.
Ronaldo, who was given the FifPro young player of the year award prior to kick-off, even managed some theatrics taking a penalty as he faked before sending Richard Lee the wrong way.
Watford's only win at Old Trafford was as a Third Division side in the League Cup in 1978 when they came from a goal down.
History was never seriously in danger of repeating itself, although Powell went close to a freakish equaliser when his deep cross caught out Kuszczak and hit the far post.
Ronaldo put United ahead from the spot
Nemanja Vidic twice went close with free headers from Michael Carrick corners and Richardson saw a shot deflected narrowly over the bar as the half came to a close.
But it took until 60 minutes for United to enjoy any kind of sustained pressure on the Watford goal, with Solskjaer, Rooney and Richardson all unable to convert.
When the second arrived it was cruel luck on Watford as Richardson's left-wing cross arrowed off the head of Lloyd Doyley and past Lee.
Lee did brilliantly to deny a further own goal from Adrian Mariappa but the third soon arrived when Larsson - minutes after coming on - was spoon-fed by Rooney and he casually slotted in his first Premiership goal.
United saved their best for last as Ronaldo's brilliant pass was controlled by Rooney before he lobbed a shot over Lee and just under the bar for his fourth goal in three games.
Ronaldo went close to a fifth when he fired against the post.
Manchester United boss Sir Alex Ferguson:
"The first half was very difficult. Watford frustrated us and they were difficult to break down. I thought the penalty might open the game for us.
"But our football improved so I'm satisfied. Watford have had a hard season and they came here with a plan.
"There was some careless finishing and some great opportunities can be frustrating and football can bite you. One-nil is not a healthy scoreline."
Watford boss Aidy Boothroyd:
"They are a world-class team, the biggest club in the world and it is a privilege to be here in the first place.
"I hoped we'd get more out of game but realistically we can't compete with the team they had out there, but we kept going and we can walk out here with our heads held high.
"My belief hasn't changed. I believe we'll stay up - coming here nothing (and losing) changes nothing."
Man Utd: Kuszczak, Neville, Ferdinand, Vidic (Silvestre 75), Heinze, Ronaldo, O'Shea, Carrick (Brown 75), Richardson, Rooney, Solskjaer (Larsson 66).
Subs Not Used: Van der Sar, Park.
Booked: Vidic, Heinze.
Goals: Ronaldo 20 pen, Doyley 61 og, Larsson 70, Rooney 71.
Watford: Lee, Mariappa, Doyley, DeMerit, Stewart, Smith (Williamson 72), Bangura, Francis, Powell (Hoskins 61), Bouazza, Henderson (Kabba 75).
Subs Not Used: Chamberlain, McNamee.
Booked: DeMerit, Henderson.
Ref: M Dean (Wirral).