Here's the official search for the ÖBB (Austrian railroad): http://fahrplan.oebb.at/bin/query.exe/en
- trains from Wien to Salzburg usually take about 2.5 to 3 hours. Most trains go directly between the two cities, no need to change trains and such. Trains are the cheapest solution - but they're a bit of a ripoff nevertheless. If you travel with a second person, you can get a somewhat cheaper "partner" ticket.
You're coming at an unfortunate time though - both the train station in Vienna (Westbahnhof) and in Salzburg (Hauptbahnhof) are being renovated/rebuilt at the moment, and there might be some more trouble than usual with the trains.