This is a sore spot for me, and it pains me to suggest anything to simulate a guitar, but I've heard good things about a program called "Guitar Rig." I also know of a Cakewalk-sponsored program called "Guitar Pro." Both of these programs may be helpful to you.
That said, I've never actually heard either of these programs, and I strongly advise using a real guitar if you can, as I've never heard a simulator that sounds convincingly enough like the real thing to not make me cringe when I hear it. I'm not saying that one doesn't exist, but I just haven't heard one. In my opinion, guitar is one of the hardest instruments to simulate convincingly, for whatever reason (though that may just be my guitar-bias shining through).
Good luck in finding a simulator, but honestly, if you can't find one that does the job, a guitarist is always available in a pinch. (Me, for example.) If you desperately need a guitar line just give me an email; I'll consider it a public service to help wean someone else off sims. :)