Roy O'Donovan scored the only goal of the game for the Cobblers after 26 minutes at the Sixfields Stadium.
Boothroyd's side had a number of chances to extend their lead but have to be satisfied with just the one goal advantage to take to Whaddon Road on Sunday.
"I thought we played very well tonight," Boothroyd told Sky Sports. "The only disappointment for me is that we didn't score more goals. But it is only half time and we know these are a good side.
"I said all along it's going to be difficult but we've given ourselves a good chance.
"We've got to go there and make sure we stand up to whatever comes in and try and see if we can give them some problems as well.
"I thought it was a terrific game, two very evenly matched teams and it will be down to the details."