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."
Kettering 12 Walker 02 P Ifil 03 Davis 05 Sangare 06 J Ifil (Noubissie - 78' ) 18 Mills 20 Bridges 30 Joyce 34 York (Ford - 46' ) 25 Wyke 29 Gray Substitutes 15 Westwood 17 Ralth 23 Noubissie 26 Ford Alfreton Town 27 Stewart Booked 02 Law (Mullan - 80' ) 03 Franklin 26 Kempson 29 Connelly 08 Moult 11 Arnold 17 Deverdics 09 Clayton (Streete - 58' Booked ) 15 Wilson (Cunnington - 66' ) 16 Jarman Substitutes 01 Lowson 04 Streete 19 Mullan 28 Cunnington