Rob Earnshaw's first-half strike was enough for the hosts who made it an impressive 11 home victories on the trot as they chase Premier League football next season.
For the visitors it was a familiar story as their Championship campaign remains punctuated by good performances but poor results.
Forest boss Billy Davies made two changes to the side that lost to Barnsley in midweek. Midfielder Paul Anderson returned to the side with George Boyd unable to play against his parent club and Dexter Blackstock was preferred to Dele Adebola in attack after overcoming a knock.
Injury-hit Posh endured a nightmare prior to kick-off and did not arrive at the City Ground until well after 2pm after the team coach failed to collect them at London Road. The matchday squad were forced to travel to Nottingham in their own cars and in turn were caught in congestion. Tommy Rowe was named skipper for the day after he was the only player in attendance when the team sheet was handed in.
Forest's aspirations of an automatic promotion berth had stalled on the back of six successive away defeats and the hosts were almost caught cold when Chris Whelpdale went close for Jim Gannon's side early on, flashing a long-range effort a yard wide of Lee Camp's goal.
But the deadlock was broken in the 13th minute when Chris Cohen's long ball to the far post was headed back across goal by Radoslaw Majewski and an unmarked Earnshaw did the rest for his 13th goal of the season.
Earnshaw would have added a second but for a fine stop from United goalkeeper Joe Lewis as Davies was forced into a reshuffle when James Perch was forced off after 20 minutes after taking a knock, Luke chambers entering the fray.
Craig Mackail-Smith, Charlie Lee and, with the best opportunity, Rowe all tested Camp as the visitors battled their way back into the game and the rain continued to fall heavily.
At the other end Blackstock headed goalwards for Lewis to save while Majewski was fortunate to escape with just a yellow card after a full-on challenge on Scott Griffiths as Forest went in perhaps fortunate to be ahead at the break.
Dominic Green and Reuben Reid were introduced at the start of the second half as Gannon looked to freshen up his side.
Camp was called back into action early on to parry firstly Gabriel Zakuani's header and then gather Mackail-Smith's effort but Forest began to find their feet.
Zakuani hammered an effort that Camp failed to hold as Posh came again but Forest stood firm and were inches away from a second when Cohen let fly from range and Lewis used his fingertips to divert the ball onto a post.
Lewis dived full-length to push sub Garath McCleary's strike away from goal and Cohen was booked for a challenge on Sergio Torres but the hosts held firm for maximum points despite a late Posh penalty shout.