Adding some digital strings/keyboard

I love my Audacity. I use it lot. Mostly to record my little compositions and to record changes to jam against. But despite having a couple hundred tunes recorded, my approach is ultra ultra low tech. Anyway, some old friends and I are going to use Audacity to do some intercontinental recording. I’m a guitar player/singer, but I’d like to add some strings/keyboard or whatever on some tracks just for kicks. Is there a free and hassle free synthesizer program that would work well with audacity on vista? I mean something that takes less than 30 minutes to learn and doesn’t require any gear (cause I don’t have anything; I just use the laptop mic!). Ideally, I could just enter the music notes and the instrument to play them and it would spit out something that I could put in Audacity?
Thanks in advance.

Have a look at MuseScore