Face it, Superman is one of the most legendary and powerful heroes ever. He's got like every good power there is. He's pretty beast.
Although much less popular than Superman, Thor the greatest of the Marvel superheroes. He's immortal, which means Superman can't "kill" him. He's also got lightning powers. And that cool hammer! Thor will definitely beat Superman down
Ok, lets say Thor uses hs powers to transport into the DC Universe, after someone convinces him that Superman is a bad guy and he's the only one who can stop him from getting into the Marvel Universe. Thor interupts Superman's daily patrol of the city, and they get into an all-out brawl over the city.
It would be one heck of a fight, that is if neither one of them stopped and realized they didn't have to fight each other. But lets forget that one...
Personally, I think it would be a long fight, but Thor is immortal, so he could eventually beat-down Superman.
But that's just me...