How can someone who just connected a MIDI keyboard via USB to a Windows 8 PC take advantage of some nice instruments?
I installed MuseScore and can enter notes by playing on the USB-connected CASIO keyboard (CTK 2080), but what I would like is to just play on the keyboard and the have computer render the sounds (not record sheet input), using for example xylophone sounds?
What you're looking for is a software instrument. It takes midi input from your keyboard, and generates audio data, either using pre-recorded sounds (in which case it would be a sampler) or generating sound on-the spot (a synthesizer).
There is a very vast repository of virtual instruments available for windows - some free, some commercial. Most of them are focused on specific instruments or categories. There are also packs of many different instruments, such as NI's Kontakt.
If you're looking for free software, a good alternative might be using soundfounts. There are dozens of free soundfonts available, only a google search away.
It's worth noticing that, although some software instruments provide stand-alone programs, many are only available as VST plugins. In order to use VST plugins and soundfonts, you should learn how to use a digital audio workstation.
I'll call this a "partial answer" that might help you get started in researching your problem.
Googling "windows 8 musescore midi input" reveals a number of posts. Try this one for starters: http://musescore.org/node/12098 . If you can't find something that helps you, then try posting on musescore.org.
