Vienna has a good subway system, and I believe that is the best way to get around. That's what I did this summer. It can be supplemented with taxis if you're going off the beaten path, or don't like to walk. Having only 48 hours, you're quite limited. I'd go to the Stephansdom and wander around there a bit, then go to the Kunsthistorisches Museum, which has an Old Master painting collection the equal of any. That will pretty much eat up all your time! If you don't like that much art, you could go out to Schoenbrunn Palace. I went to the Torture Museum, which was campy and fun. It's in an old WWII bunker, and they have it rigged with sound so you can pretend you're undergoing a bomb raid. Two attractions for the price of one!

Learn some German if you don't know any. Your hotel people will speak perfect English, but that's not always the case elsewhere. Less English spoken than in Germany.