It looked like Aidy Boothroyd's men might be going into the international break in one of the relegation places after Sean St Ledger fired the visitors into an early lead.
But by the midway point of the first half, Watford had turned the game on its head with goals from Jon Harley and Smith.
Having taken the lead, the Hornets went on to play some attractive attacking football for the remainder of the first period and probably deserved to by further in front by the interval, but in a disjointed second half from both teams it was a case of getting the job done for the home side.
The visitors went in front in just the fifth minute when a Ross Wallace cross from the right was headed back into the heart of the penalty area by St Ledger and flicked on by Neil Mellor.
The ball then rebounded towards Adrian Mariappa, who made a complete hash of his attempted clearance, leaving St Ledger to curl the ball beyond Richard Lee from eight yards.
Watford have developed a habit for making life difficult for themselves of late, but their response was terrific.
Within seven minutes they were level when John-Joe O'Toole lifted the ball into the path of Harley on the left side of the area and he struck a first-time shot back across Andy Lonergan and inside the far post to equalise.
And nine minutes later, an excellent early pass from Will Hoskins sent Smith beyond St Ledger and, although Lonergan saved his first effort, the Preston keeper was helpless to prevent the winger from reaching his half-century for the Hornets.