Manchester United moved into the fifth round of the FA Cup with a professional win over Northampton at Sixfields.
The visitors recovered from Diego Forlan's early penalty miss to take a first-half lead through Mikael Silvestre's scrambled effort.
A Chris Hargreaves own-goal just after the break killed off any hopes of a Cobblers comeback before Forlan added a third.
But Northampton played their part in an entertaining tie and Marc Richards saw his volley thunder off the bar.
United signalled their intent from kick-off, swiftly launching attack after attack.
And the Premiership club were rather fortuitously awarded a penalty just two minutes into the game when Chris Carruthers made minimal contact with Ronaldo, who had jumped to meet Quinton Fortune's cross from the left.
3 mins: Forlan penalty saved by Harper
18 mins: Asamoah causes panic in the United area
34 mins: Silvestre forces home the opener for United
47 mins: Hargreaves nets own-goal
68 mins: Forlan nicks a third
The Portuguese attacker did enough to convince referee Steve Bennett that he had been fouled and Forlan stepped forward to take the spot-kick only to see his weak effort saved by Lee Harper.
United had plenty of pace up front with David Bellion, Forlan and Ronaldo running the Northampton backline ragged.
But without top scorer Ruud van Nistelrooy, the visitors lacked a real cutting edge and they squandered several chances to take the lead.
Ronaldo, who has an interesting ability to produce the sublime and the ridiculous in equal measure, whipped in an inviting cross for Bellion.
But the Frenchman somehow failed to make decent contact when it looked easier to score from two yards out.
The Cobblers challenged for everything
Although United dominated possession, the Cobblers - in the shape of the diminutive Derek Asamoah - did cause some awkward moments for Sir Alex Ferguson's men.
Roy Carroll had to race off his goalline to block Asamoah's shot after the United rearguard had been caught square.
However, the United tidal wave continued unabated and Silvestre bundled home the opener after Northampton's defence had failed to deal with Ronaldo's initial shot.
After the interval, the Cobblers suffered a double setback. Hargreaves turned the ball into his own net from Ronaldo's corner and Carruthers was stretchered off with a neck injury.
It was hard luck on Northampton who, despite the 82 league places that separate them from their more illustrious guests, would have felt that at 1-0 they still had a chance to draw level.
The woodwork came to United's rescue on the hour when Richards' dipping volley crashed against the crossbar.
Asamoah hit the post late on and Carroll was forced to make several smart saves as Northampton searched for a consolation.
Forlan made amends for his earlier penalty blunder by converting from close range after Ronaldo headed back across the face of goal.
But anyone hoping for a repeat of the 8-2 drubbing that United inflicted on Northampton in the 1970 FA Cup was always going to be disappointed.
For despite United's superiority, not one of their players at Sixfields could match the genius of double hat-trick hero George Best on that cold February afternoon 34 years ago.
Northampton: Harper, Reid, Sampson, Willmott,
Carruthers (Chambers 51), Lyttle, Hargreaves,
Lincoln (Harsley 72), Smith, Asamoah, Reeves (Richards 45).
Subs Not Used: Thompson, Vieira.
Man Utd: Carroll, O'Shea, Brown, Silvestre (Bardsley 72),
Fortune (Pugh 89), Bellion, Fletcher, Butt,
Scholes (Richardson 72), Ronaldo, Forlan.
Subs Not Used: Howard, Keane.
Referee: S Bennett (Kent).