Dave Tregurtha headed the first from a set piece after five minutes, and although the home side levelled through Fitzgerald midway through the half, United were soon back in front when Chris Bangura pounced on a defensive error to round the keeper and score.
The Boss, forced into action as a substitute after Joe was forced off through injury, sent the travelling support wild when knocking home Dwain Clarke's cross for the third goal just before the break.
Job done, Hercules replaced himself with Darren Bray, and as against Bedford the week before, United efficiently killed off the second half.
They tired towards the end of the game, and Northwood came close to a second goal, but United held on for a deserved win and only their second league double of the season.