I far as I'm aware the only dedicated Violin and/or Piano simulators would be VST instuments or 'soft synths' that are triggered by midi - rather than the VST effects (which are applied to recorded audio) that you are looking for....
The easiest and cheapest way get pianos and violins and sound effects and textures into your recordings would be to download something like
Multitrackstudio then...
Record your guitars/ vocals in Kristal or Audacity > save as WAV > export the track to multitrackstudio and record the pianos etc, using VST instruments.
The free Multitrackstudio download limits you to 3 tracks so you'll sorta have to do it this way...
You'll find free piano', string and choir instruments
here but a midi keyboard is a really a must for playing in realtime.
hope that helps
Mick