Does anyone know of a string simulation program that can create violin/viola/cello sounds that can be manipulated?

I always recommend getting a soundfont player ( sfz is a great free vst) and loading up on free soundfonts.

If you’re looking for a PROGRAM, then Fruity Loops is a fairly good one to use (Kind of pricey). Then you have to implement the SOUNDFONTS (those actually make the noises you want).

If you have the money to put out for some good soundfonts, I suggest Personal Orchestra.