Well, if you are looking for a "ready to go" free program, I don't know of any; however, you can get some fairly convincing real sounding strings out of Superwave P8 and TranceDrive.
After I read this post, I decided to take it upon myself to make a preset that would sound like the Orchestral Harp MIDI Soundfont I have; it came out surprisngly very close.
So, here is something you might find useful: Superwave P8 Harp Preset
I just started tuning the synth sound until I could get it as close as I could to the original. I couldn't quite capture the "punch" that comes from an actual string being plucked, but once I added reverb, I would say that they are nearly the same sound - just the synthesized version sounds a bit softer.
please note that I am comparing a 44.1kHz sample to a 192kHz synthesized sound. There is a noticeable difference if I render at a lower sample rate.
So, my advice to you would be to get a program like P8 that gives you lots of waveform and filter control and learn how to use it by trying to recreate sounds. P8 isn't the best for sure, but it is free and it is one of my bread and butter synths I use.