The Pilgrims’s home form has been their Achilles heel for much of the season, but they led from the first minute to the last in a game they comfortably dominated.
Jake Wright needed just 36 seconds to open the scoring before Max Wright and Ben Davies got their names on the scoresheet in an exciting first half.
Lewis Reilly’s textbook free kick gave the Nash some hope, but only for a matter of minutes, Tom Crane finishing off the scoring at the death.
Such is the tight nature of the division that Boston’s three points mathematically secured their survival in a campaign where they have never dipped below 12th.
With Gavin Allott unable to get over his thigh injury, the only change to the side which drew 2-2 at York City last weekend was Jake Wright, joining Jay Rollins in attack.
Making way was Andi Thanoj, dropped for the first time since joining the club more than a year ago. There was no place in the squad for deadline-day signing Cameron Hawkes.
It took just 36 seconds for Jake Wright to open the scoring, making the most of a defensive error as two Curzon defenders went for the same ball and both failed to make contact, the striker sneaking in to flick beyond Cameron Mason.
Boston opened at the scoring at home for the first time since their 2-0 win over York in December, nine contests ago.
And it was 2-0 in the 24th minute as Max Wright scored his second in as many games.
Meeting George Smith’s whipped cross his scuffed effort had too much pace for Mason. Not as pretty as the finish seven days earlier, but just as effective.
In between those two goals United pressured the nervous-looking Ashton defence, Jake Wright firing wide and Ryan Qualter’s looping header landing on the roof of the net.
Rollins’s downward header and Davies’s free kick into the chest of Mason came close to adding to the advantage before the away side pulled one back with their first meaningful effort.
Reilly’s 41st-minute free kick was a picture of perfection, looping over the United wall and beyond the dive of George Willis.
But Davies restored the two-goal advantage three minutes later with a thumping penalty, his eighth goal of the season, after Nicky Walker was tugged back by Olly Thornley.
The second half began with Mason pounding away two Max Wright long rangers.
The Nash looked to sneak their way back into the game with rare forays forward and, from one such, Thornley had space at the back post but at full stretch looped over the bar.
But it was much closer than Reilly’s 20-yard effort which cleared the ground.
Curzon were down to 10 men for the final 15 minutes as Sean Miller pulled up injured after all three substitutes had been made, allowing the hosts to go through the motions until a late spurt of action in stoppage time.
Willis tipped away John McAtee’s drive before Clare forced home Ashley Jackson’s cross with virtually the final kick of the game.
UNITED: Willis, Davies, Smith, Wroe, Cresswell, Qualter, M. Wright, Abbott, J. Wright (Thanoj 88), Rollins (Jackson 89), Walker (Clare 80); Subs (not used): Wafula, Johnson.
CURZON: Mason, Senior, McJannet (McAtee 62), Hunt, Thornley, Rowney, Miller, Brooke (Shaw HT), Reilly, Shenton (Morgan 74), Wall; Subs (not used): Wardle, Deakin.
REF: Elliott Swallow.
ATT: 1,406 (39).
STAR MAN: Jake Wright - A constant pain in the neck for the Nash, hurrying, hassling and causing problems.