The little Canadian, who left Bramall Lane 13 months ago, came off the bench to carve out the goal and dent United's hopes of recapturing a play-off spot.
Peschisolido, who had a habit of scoring from the bench during his time at United, chased Grzegorz Rasiak's through ball, keeper Paddy Kenny could only block his shot and Bisgaard fired into an empty goal.
High-flying Derby had carved out the chances . Tommy Smith was foiled by the post on 12 minutes after wriggling past his marker to finish with a stinging left-foot strike from 20 yards which crashed off the woodwork with Kenny beaten.
Inigo Idiakez and Bisgaard tried their luck with a long-range efforts but each time the ball skidded inches wide of the target.
Derby, playing the better football on a slippery surface, kept United at bay until the 37th minute when Alan Quinn caused problems on the left flank.
The ex-Sheffield Wednesday midfielder picked out on loan Watford striker Danny Webber, who slipped his marker before blasting the ball high over the target from 14 yards.
Bisgaard almost found a way through in the 56th minute but the hard-working Nick Montgomery tracked black to block his low shot.
Marcus Tudgay found space on the right minutes later sweeping past full-back Derek Geary but United skipper Chris Morgan steamed in to block his cross with Rasiak waiting to pounce.
United midfielder Phil Jagielka caught out the stubborn Derby back four in the 61st minute but after his close-range header was deflected he sliced a follow-up shot wide.
Quinn tested Lee Camp with a spectacular left-foot volley before Peschisolido came on to replace Tudgay and set up the decisive goal.