Jay DeMerit's header sank Blackburn to give rock-bottom Watford only their second Premiership win of the season.
The home side took the lead when Brett Emerton nodded Tommy Smith's cross into his own net at the far post.
Blackburn levelled when David Bentley made a jinking run down the right and crossed for Benni McCarthy to head home via the underside of the bar.
But Watford kept pressing and DeMerit gave them the points from close range after a free-kick was not cleared.
Interview: Watford boss Aidy Boothroyd
It was the Hornets' first league victory since 4 November and they are still seven points adrift of safety - and with a lot of work to do.
Manager Aidy Boothroyd had seen Ashley Young seal his £9.65m move to Aston Villa in the hours leading up to kick-off and it is clear some of those funds need to be invested in the squad if they are to retain any hope of staying up.
They did deserve this success but sadly the determination displayed by Boothroyd's men was rarely matched by any of the other qualities they will need to show to survive.
Yet against a supposedly in-form Blackburn side - who had won five out of their last six matches but looked strangely off-colour here - effort alone was enough.
Watford took the lead on 12 minutes when Smith's cross looped to the far post and, in trying to clear, Emerton only succeeded in beating Brad Friedel.
That mistake aside, the visitors coped well with the barrage of high balls being pumped into their box.
But they had created little themselves apart from an opening for McCarthy that the South African sliced over and a 30-yard effort from Bentley that fizzed wide.
606 DEBATE: Your thoughts on the game
It was largely uninspiring stuff - but out of nothing Bentley fashioned a superb equaliser just before half-time.
He beat two men with a jinking run down the right wing before crossing for McCarthy to plant an unstoppable header into the net via the underside of the crossbar.
Sadly for Mark Hughes' men Bentley was about their only player looking remotely dangerous.
And at the other end the home side were growing in confidence with Jordan Stewart testing Friedel from long range and Darius Henderson going close with a header.
Shabani Nonda should have done better for Blackburn when the ball broke to him in the box but after Robbie Savage was stretchered off Watford began to take full control.
So it was no surprise when substitute DeMerit grabbed the winner with 20 minutes to go, stooping to send the ball into the far corner after the ball had flown around the Blackburn box.
Blackburn at last attempted to up the ante but Watford were not about to let go of the points they had worked so hard to secure.
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Watford boss Adrian Boothroyd on his side's trip to the cinema before the game rather than a normal training session:
"We don't win many games so I have got to be as creative as I can.
"The cinema opened up for the lads to watch Rocky Balboa and I worked with the reserves.
"They had heard enough of me so it was time for them to hear someone else. Maybe they should go to the pictures more often."
Watford: Foster, Mariappa, Mackay, Shittu (DeMerit 33), Stewart, Smith, Bangura, Francis, Bouazza, Henderson, Hoskins (Powell 85).
Subs Not Used: Lee, Priskin, Ashikodi.
Booked: Stewart, Bouazza.
Goals: Emerton 12 og, DeMerit 70.
Blackburn: Friedel, Emerton, Henchoz, Khizanishvili, McEveley, Bentley, Savage (Gallagher 57), Mokoena, Pedersen, Nonda, McCarthy (Derbyshire 69).
Subs Not Used: Enckelman, Peter, Nolan.
Goals: McCarthy 45.
Att: 13,766
Ref: D Gallagher (Oxfordshire).
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