People always tend to say that Wenger spotted Henry, like he did with Fabregas, Eboue etc. He got him for 10 million and he was already a decent winger at Juventus. Wenger worked wonders with him at Arsenal, but he didn't exactly spot the talent.
About Ronaldo, United have an affiliation with Sporting Lisbon, which generally means we get first option on their players. He wanted to join a big club, with history and recent trophies, and a large stadium. The only one with all of them is United.