Don't bother with the Zeitgeist crap - and if you want to know what we think of Zeitgeist, it's sourcebook, etc, head over to the Atheist Experience blog on Freethought Blogs and look it up. Zeitgeist offers some of the worst reasons ever to be a atheist.
If you want a scholarly treatment of early influences on Christianity, read something by Richard Carrier. He's written books (I recommend "Not the Impossible Faith"), but a lot of his stuff is on www.infidels.org. He's also on Freethought Blogs, where he recently thrashed Bart Erhman for some of Erhman's misrepresentations of the mythicist position.
In short, there are some very good reasons to believe Jesus was a mythical (vs. historical) figure, but you won't find them in any of the Zeitgeist garbage.