The Iceland international returned this season on loan from QPR and showed his ruthless streak in front of goal.
A brace on his second coming against Leicester in September was tempered by a calf injury that has sidelined him since.
But he returned with a crucial ninth-minute strike to set Watford on their way to a crucial win.
Helguson is just one of four canny loan signings by Malkay Mackay, along with Arsenal midfielder Henri Lansbury and Manchester United pair Craig Cathcart and Tom Cleverly.
Lansbury and Cleverly - a potent combination all afternoon - combined for Watford's killer second in the 33rd minute.
Mackay said: "Heidar gives 100 per cent. It's nice to get someone over 25 on the pitch that the youngsters can learn off."
Preston have picked up one win in eight and boss Alan Irvine moaned: "We were poor. We didn't match their energy or movement."
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