The U's played for almost an hour with 10 men after Kem Izzet's red card in Boothroyd's first game in charge, but held out for a goalless draw to frustrate Alan Pardew's Saints.
The former Watford boss said: "To go to a place like that and get a point feels like a win to me.
"When the whistle went I thought 'this could be the start of something really big'.
"I have always been the underdog where my teams punch above their weight and I think we can do that here as well.
"It's always good to prove people wrong and after a nice break I've got the hunger for it again."