NOTE: This is an older article, from before the time Finale or Sibelius hosed AU or VST plugins. If you're using Sibelius 5 (or higher), or Finale 2010 (or higher), this article most likely does not apply to you.
If your computer freezes when you select a note in Finale or Sibelius or when you try to play back your score, you probably have your MIDI-IN and MIDI-OUT settings configured incorrectly. By this point, we'll assume you already have your virtual midi cable installed and activated (MIDI Yoke, Maple MIDI, IAC driver, etc.). If you're not sure, stop here and download the Quickstart Guide (attached below) to setting up VDL:2 within Sibelius or Finale. If you're just getting started, you'll save yourself some grief by reading carefully and going through each step one-by-one.
If you are still crashing, check the following:
In Sibelius (or Finale), check to ensure that your playback device (midi output, i.e., MIDI Yoke 1) is NOT also selected as your input device. If this happens, you will create a midi loop which will likely freeze your machine. A midi loop is kind of the same thing as when you aim a microphone toward the speaker, creating feedback. Not good.
Your INPUT DEVICE in Sibelius (or Finale) should be set to your input keyboard (your midi keyboard). If you aren't using a midi keyboard, make sure that you select an input device OTHER than what you have set as your output device or, alternatively, ensure that nothing is selected as the input device.