On the podcast of yesterday's show, Greg responds to the claim that the "myth" of Jesus was created by piecing together older myths, an idea made popular by the internet sensation, Zeitgeist. Greg identifies the main problem with Zeitgeist's argument and gives a couple good illustrations that will help you explain to your friends why the existence of other myths, even if similar, does not prove the claim that Jesus is a myth. (You can read about the book Greg mentions in the podcast, The Wreck of the Titan, along with a response to some of the specific claims of similarities between Jesus and Mithras on PleaseConvinceMe.com.)
If your friends have been asking you about Zeitgeist, there are various other rebuttals posted on the internet that might help. But I found one that I think is particularly effective, even if it doesn't go into detail. It's posted on the blog of an atheist call-in television show. Yes, an atheist show. So many of their fans were asking what they thought of the movie that they posted their stock response on their blog. Here's an excerpt:
Most of us have seen Zeitgeist and we've commented about it numerous times on both shows. I've actually watched it several times, and if others hadn't already done a brilliant job of debunking the nonsense in that film, I'd probably devote more time to doing exactly that.
The first third of the film is an unscholarly, sophomoric, horribly flawed, over-simplification that tries to portray Christianity as nothing more than the next incarnation of the astrologically themed religions that preceded it. Like all conspiracy theories, they combine a few facts, focus on correlations and build an intriguing story that seems to fit the pieces together nicely - provided you don't actually dig below the surface to find out where they might have gone wrong.
The label "unscholarly nonsense" has a lot of credibility coming from atheists who have nothing to gain. Even if this doesn't convince your friends outright that Zeitgeist is ridiculous, they may go on to research the topic with a much more open mind.