Hornets boss Adrian Boothroyd fielded five of his summer signings, including striker Darius Henderson and defender Clarke Carlisle, both of whom arrived at the club in the 48 hours prior to the game.
But Henderson got his Hornets career off to the perfect start when he connected with an Ashley Young cross after nine minutes and guided a simple header past Carlo Nash.
On the 22-minute mark the visitors levelled after some poor defending from the Hornets. Jordan Stewart was turned far too easily by Chris Sedgwick and he slipped a pass through to Dave Nugent.
The striker, who is one of the rising stars of the Championship, attacked a retreating Hornets defence before striking a low right-footed shot that caught Ben Foster totally unawares, although the keeper may have been unsighted.
Stewart also had to shoulder a large share of the blame for what was to turn out to be Preston's winner after 37 minutes. The former Leicester midfielder, playing at left-back was shaken off the ball with ease by Dickson Etuhu who advanced into the Watford box before slamming a shot past Foster.
After a lively and enjoyable first half more goals looked inevitable in the second period. But the game became a scrappy affair, punctuated by the award of numerous free-kicks by referee Iain Williamson.
Watford rarely looked like they would find a way through and equalise, although substitute Hameur Bouazza should have done better with four minutes left when he headed over from six yards out following a Lloyd Doyley long throw.
Preston always looked capable of upping their game if necessary, but the closest they came to adding to their tally was late on when Paul McKenna was released in the area only to shoot straight at Foster.
The defeat brought more disappointment for the Watford fans, who only saw their side win five of their 23 home league games last season.