The Blades had the first chance after four minutes when Darius Henderson on the right found Jamie Ward on the left and from inside the box he spun and hit a low shot which was parried.
Ross Wallace had a right-wing in-swinging corner to the near post saved.
Preston opened the scoring in the 21st minute when a long right-foot free-kick by Billy Jones slipped off Nick Montgomery's head and Sean St Ledger chested the ball down and volleyed it from close range into the bottom right corner.
Brian Howard lobbed the ball over the defence to Stephen Quinn and from eight yards out his shot was saved and the resultant in-swinging right-wing corner by Quinn was saved by Paddy Kenny.
In the 35th minute, Jon Parkin had a a close-range header that hit the right-hand post from a Wallace right-wing in-swinging corner to the near post.
Neil Mellor played a one-two with Darren Carter and had a left-foot shot saved and before half-time, Brian Howard had a right-wing corner saved.
Within 20 seconds of the restart, United equalised when the Preston keeper saved from Henderson but Jamie Ward assisted with a right-wing cross and Howard hit a left-foot volley into the empty net from close range, high into the middle of the goal.
Howard had an out-swinging corner saved and Chris Morgan had a shot saved.
Eddie Nolan found substitute Chris Brown and his flicked header was saved.
Howard had 25-yard volley saved one handed, before Kyle Walker, Howard, James Beattie and Walker again all had efforts saved.