Carew has had injury niggles this season and during the past couple of months has been used mainly as an impact player from the bench.
But the Norway international capitalised on his start against Crystal Palace in Wednesday night's FA Cup fifth round replay by winning - and scoring - two late penalties to set up a last-eight meeting away to Reading on March 7.
Now Carew is hoping he can play a part at Wembley in this weekend's meeting with Sir Alex Ferguson's side, from whom Villa have taken four points in the league this season.
He said: "I've been injured and I've been working really hard to get back in shape and against Palace I felt really good - even after the game. My legs felt light.
"I feel I'm starting to reach my peak form again. I really needed that game to get the last bit of fitness back after a couple of setbacks.
"I want to be available for the team again and 100% fit."
Carew added: "This is a really important stage of the season. We are in two cups still and it's great for the club.
"We have reached one final and we can reach another final.
"It's just good to be back and be ready for the last few weeks. As regards the final on Sunday, I always hope to play but I am just available for the team."
Carew does not believe Villa should be fazed by the prospect of meeting the Red Devils in what will be their first major final since the 2000 FA Cup, when they were beaten by Chelsea.
He said: "In the league, we have taken four points off United so we know we have the possibility to get something out of it.
"We are going there planning to enjoy the moment, play good football and try to make it a great day in Villa's history. That would be fantastic.
"To win the cup would be unbelievable. Both teams will want to win it but we have to be confident and we don't have to be scared that we can't compete with Manchester United."
Carew feels Villa have made steady progress under O'Neill but reaching a final is another crucial phase in their development.
He said: "We have been getting better and better in the league over the last three or four years, which is important, but it's also important to get to cup finals.
"You really feel you are arriving. It gives even more edge to the feeling you are getting somewhere.
"It is not many times in your life that you get an opportunity to play in a game in front of 80-90,000 people. We must enjoy the moment and be confident we can win."