When a christian is doing a translation of the Jewish texts, they tend to look at the "old testament" as something that is predicting Jesus, and chose words to try to emphasis that (including some out and out mistranslations and lies, such as Psalm 22, and isaiah 7:14).
Isaiah 7:14 (KJV):
Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
Isaiah 7:14 (Jewish Translation from the Hebrew)
Therefore the L-rd Himself will give you a sign: Behold, the young woman is with child, and she shall bear a son, and you [or, she] shall call his name Immanuel.
The NIV, ASV, and a few other "Christian" translations of the Bible agree with the KJV and use "virgin"; the RSV, Catholic JB and NJB, and others use "young woman" or something similar.
See
Forbidden ("Is Isaiah 7:14 a Messianic Prophecy?") for a more detailed discussion.
I tend to agree with the "young woman" reading simply because it fits the context of Isaiah better. The purpose of the young woman having the child was to give a message to King Ahaz about how soon God would kill the Kings of Aram and Israel, and how he would later bring destruction upon Judah. (It all seems a bit complicated to me, but I'm not God, so I don't know all the ins and outs of His reasoning.)