Despite making a positive start, North End - with just one win in seven games - fell behind in the 13th minute when Andreas Weimann, on loan from Aston Villa, got on the end of a cross from Danny Graham to fire past Iain Turner.
The home side levelled, though, in the 28th minute when Barry Nicholson`s 30-yard effort eluded Hornets goalkeeper Scott Loach.
Preston moved in front 11 minutes into the second half when Iain Hume notched his 12th goal of the season with a 25-yard free-kick.
Phil Brown`s side wrapped up the points with two minutes left when Jamie Proctor claimed his first professional goal from close range after being set up by Keith Treacy.
Source: PA
Source: PA