Manchester United overcame Derby's resilience to move within a point of Premier League leaders Arsenal.
Giggs and Tevez were both on target for United
Ryan Giggs put United ahead with his 100th league goal, slotting home after Ronaldo's shot was only parried.
Carlos Tevez made it 2-0 from a Ronaldo free-kick before Wes Brown teed up the Argentine striker for a third.
Wayne Rooney and Giggs hit the woodwork before Steve Howard bundled home a late reply for Derby but Cristiano Ronaldo's penalty sealed a fine United victory.
It has been a dismal season on the road so far for the Rams but boss Paul Jewell can at least take encouragement from Howard's strike, even if the big striker seemed to know little about it.
It was Derby's first goal in seven games and their first away from home this season. It was also Howard's first of the season for good measure.
For more positive signs, Jewell can also point to the way his side adapted well to the treacherous conditions at Old Trafford and, although they failed to create any clear-cut openings, kept United quiet early on.
When United did attack they found the visitors in determined mood.
The Rams' attitude was exemplified by a brilliant Jay McEverley tackle which denied Wayne Rooney as he shaped to convert Ronaldo's cross.
But Derby's lack of a serious attacking threat was always going to limit their hopes of upsetting the champions.
And United's midfield, which included Michael Carrick as a late replacement after Owen Hargreaves was injured in the warm-up, gradually took a grip on the game.
Sir Alex Ferguson's men still had to wait until the 40th minute to take the lead through Giggs' opportunistic strike but they quickly had the game sewn up.
After Rooney had hit the post with a cheeky chip, Tevez added a second goal - firing home at the second attempt when Ronaldo's free-kick was not properly cleared.
Derby did not give up and Giles Barnes' shot gave United keeper Edwin van der Sar problems before Claude Davis tried his luck with an acrobatic effort from the resulting corner.
United were still looking dangerous though, and Giggs hit the post with a volley from Wes Brown's cross before Tevez made it 3-0 with a powerful finish.
Howard got on the end of Tyrone Mears' cross to pull a goal back for the Rams but it was United who had the last word.
Ronaldo, who was booked for diving against Fulham earlier in the week, tumbled in the area under challenge from Mears and got up to convert the spot-kick for his 14th goal of the season.
Manchester United manager Sir Alex Ferguson:
"It was a big win. Things can happen in these conditions, they didn't suit us.
"Carlos Tevez is making a great contribution this season.
"Cristiano is our regular penalty-taker so I can understand why he wanted to take it but it would have been great for Carlos if he had scored a hat-trick."
Ferguson added: "I am surprised there was no announcement about Ryan's milestone.
"Maybe no-one knew but I certainly did. It is a fantastic achievement, absolutely marvellous."
Derby manager Paul Jewell:
"We did well in the first 25 minutes and kept them quiet.
"But the disappointing thing was that we gifted United too many goals. We are not going to win any matches like that.
"We have to get better but I will take the positives and at least we scored a goal and showed a bit of courage and conviction in the second half."
Man Utd: Van der Sar, Brown, Ferdinand (O'Shea 71), Vidic, Evra, Ronaldo, Carrick, Anderson (Fletcher 62), Giggs (Saha 64), Rooney, Tevez.
Subs Not Used: Kuszczak, Nani.
Goals: Giggs 40, Tevez 45, 60, Ronaldo 90 pen.
Derby: Bywater, Griffin, Moore, Davis, McEveley (Howard 46), Leacock, Fagan (Mears 46), Oakley, Pearson, Barnes (Teale 61), Miller.
Subs Not Used: Price, Earnshaw.
Booked: Davis, Griffin.
Goals: Howard 76.
Ref: Chris Foy (Merseyside).