Harewood emerged with the glory for Hammers, grabbing a deserved lead six minutes before the interval but Teddy Sheringham was targeted by Blades in a bruising off-the-ball war after he went down in search of a penalty midway through the first half.
If Harewood had maintained his first-half threat, West Ham would have ended their run of three consecutive league defeats with a place in the fifth round, but the scuffling nature of the contest suited Neil Warnock's team and Phil Jagielka headed an equaliser after 57 minutes - thanks to a blunder by Hammers keeper Stephen Bywater.
Powerful striker' Harewood's participation was in doubt with his wife heavily pregnant but the situation certainly did not affect his game as he led the Hammers line with strength and style.
West Ham looked much the more creative side in the first half against a side who beat them comfortably at Upton Park less than a month ago - and Harewood was always the man who most threatened to deliver.
After an early sighter he sent wide the striker set up an opening by expertly chesting down Thomas Repka's cross for Hayden Mullins to fire off target.
Two minutes later Harewood stung keeper Kenny's hands from a Malky Mackay through-pass.
When Nick Montgomery struck Blades' first shot of note it was well off the mark but in the 26th minute the industrious Michael Tonge was much closer, cutting in from the left and grazing the outside of the far post with his effort.
Harewood, though, was furious when Danny Cullip brought him down on his way through on the half-hour. Referee Dermot Gallagher gave a free kick but only the mildest of rebukes to the defender.
Sheringham was verbally abused by Sheffield fans after Chris Morgan collapsed in a heap but once more Gallagher took no action. And teenager Mark Noble scuffed a good chance wide almost immediately afterwards.
But in the 39th minute, Harewood showed him how to do it with an unstoppable finish from Sheringham's pass.
And he almost added a second in first half stoppage-time, powering away from Bywater's throw following a Blades corner he was just about squeezed out by weight of numbers.
Yet West Ham started the second half as though they were struggling and the Blades began to take control.
Sheringham needed a long spell of treatment after an off-the-ball incident before Jagielka headed an equaliser from Andy Liddell's cross in the 57th minute.
Bywater looked as though he would save it but the ball crept under his body and slipped over the line.
West Ham were never again able to exert their earlier supremacy and although it was Sheffield who got stuck in it was Alan Pardew's side who picked up the bookings for wild responses with four yellow cards.