Alfreton grabbed a crucial win in their battle against relegation from the Blue Square Bet Premier but Kettering now sit seven points off safety.
Darran Kempson opened the scoring with a header from Nicky Deverdics's corner after just four minutes.
And six minutes later Jake Moult fired home a 25-yard free-kick to double their lead.
Danny Mills had the Poppies' best chance but his close-range header hit the post.
Alfreton's third league win in a row lifts them five points clear of the bottom four.
VIEWS FROM THE DRESSING ROOM
Kettering Town caretaker manager Ashley Westwood told BBC Radio Northampton:
"We'll be training tomorrow. Sometimes you get a Sunday off if you get a good performance. But that was not acceptable.
"The performances we have had since I have been here have been better than that.
"We can't afford to dip that 4, 5 or 6% in performance levels which we did today and it has cost us."