We know that there was an irregularity about Miryam's pregnancy with Yeshua. An obscure reference in the Talmud (which of course does not refer to Him by name, to avoid persecution) indicates that it was thought that He was the bastard son of a Roman soldier (sorry, I don't remember the citation off the top of my head). Likewise, in Yochanan's (John's) Gospel account, we find some Pharisees who were none too pleased with what Yeshua was saying to them responding, "We are not born of fornication; we have one father, even God" (8:41). It's not a leap to see what they were implying.
If one trusts the Gospel writers merely as men attempting to write down what they believed to be the truth (without getting into the issue of inerrancy), and if one believes in the Cross and the Resurrection, then it is not difficult to accept the virgin birth. If one doesn't, then it sounds like foolishness--which is why no Christian apologist starts with trying to prove the virgin birth; it simply does not have the secular support (nor should we expect it to) that the Resurrection does.
If you're honestly trying to study this, my advice is to start with the Crucifixion (attested to by secular historians like Tacitus) and Resurrection (which was undeniably accepted by the whole Church right from the beginning, even in Jerusalem where it would the the simplest of matters to disprove) and with those aspects of the Gospel accounts, Acts, and Epistles which can be tested against secular history (to prove their general reliability). Then, once you've estabished those, work forward from the Messianic prophecies contained in the Tanakh (aka the Old Testament) and see if they really were fulfilled.
If you're looking for some reading material, go with Lee Strobel's The Case For Christ if you want something light and Josh McDowell's Evidence That Demands a Verdict if you want nothing but a ton of facts and quotes in the driest arrangement possible. :^)
God bless,
Thanks for all your info on this thread!