Fortune's excellent 12th-minute volley put United ahead and Cresswell headed home the second shortly before the break - just minutes after Bunn had kept out Gylfi Sigurdsson's penalty, which came after a foul on Shane Long.
And Bunn made further saves to deny Ivar Ingimarsson and Long as United held on, before Chris Morgan stabbed home with 10 minutes left to put the game to bed.
The win sees United maintain their play-off place with just one defeat in their last 11 games, while Reading will hope their FA Cup adventure does not prove a distraction as they remain in dire relegation straits.
United started the brighter and Cresswell wasted a fine opportunity to open the scoring when he blazed Stephen Quinn's cross over the bar, before Jamie Ward's stinging volley was palmed away by Adam Federici minutes later.
And the Blades went ahead from the resulting corner kick as, after the ball was initially cleared, Nick Montgomery put it back into the area and defender Fortune superbly volleyed into the top corner from 10 yards.
Reading had shown very little of the form that has seen them beat both Liverpool and Burnley in the FA Cup and they were not offering much going forward.
But on the half-hour mark they created their best chance of the match when Ingimarsson's free header was tipped over the bar by Bunn.
This chance sparked Reading into life and after an impressive 10-minute spell, they were handed a way back into the game when Long was hauled down in the penalty area by Morgan.
But the Royals could not capitalise as United were reliant on Bunn again, superbly turning away Sigurdsson's low spot-kick to keep his side ahead.
And the home side made Reading pay just two minutes later by doubling their lead.
A fine run and shot by Kyle Walker was parried by Federici and Cresswell was left with the simplest of tasks to nod home his fifth goal of the season.
Reading started the second half looking for a way back into the game and they almost found it, but again came up against an inspired Bunn, who got down well to block Long's effort.
Long-range efforts from Jem Karacan and Brian Howard did not test Bunn and as the game ran its course, Reading ran out of ideas, with United putting the game beyond doubt in the 80th minute.
Quinn whipped in a free-kick and Ward's glancing header was goalbound before Morgan turned up at the back post to poke home and steal the goal off his team-mate.