Shouldn't be a problem...
but the cranks are likely shorter (but not necessarily), the rear derailleur is probably meant for a smaller cogset, and damn, you ain't kiddin' that you got an older mountain bike if it's still got a freewheel (and will likely have a smaller cogset than most mountain bikes)....the other general thought is that road bikes aren't meant for the same level of abuse as mountain bikes but better quality parts are better quality parts on both sides of the equation...good luck.
but the cranks are likely shorter (but not necessarily), the rear derailleur is probably meant for a smaller cogset, and damn, you ain't kiddin' that you got an older mountain bike if it's still got a freewheel (and will likely have a smaller cogset than most mountain bikes)....the other general thought is that road bikes aren't meant for the same level of abuse as mountain bikes but better quality parts are better quality parts on both sides of the equation...good luck.