"Behold, the virgin shall conceive and bear a son and shall call his name Immanuel."This is almost certainly the case not because of their original context in Isaiah but because Matthew quotes them in the first chapter of his gospel to refer to the birth of Jesus. However, if we read Isaiah 7 as a whole we can see clearly that this prophecy is not one that Isaiah expected to be fulfilled hundreds of years later. It was a prophecy for his own time. Isaiah was speaking about a child that would be born very soon.
Isaiah 7 begins by telling us that King Rezin of Syria and King Pekah of Israel marched against Jerusalem the capital of Judah where Ahaz was king and Isaiah a prophet. Isaiah doesn't give us any details as to why the kings of Israel and Syria wanted to make war with Judah but considering the historical context of the time we can make a guess. The mighty nation of Assyria was a threat to all of three these nations but was likely a more immediate threat to Syria and Israel due to their geographic location. So its seems likely that Rezin and Pekah were trying to intimidate Ahaz in to forming a political and military alliance with them against Assyria, something Ahaz had been hesitant to do.
Whatever the reason for their advance, Isaiah tells us that Ahaz and the people of Judah were exceedingly fearful, their hearts were shaken "as the trees of the forest are shaken by the wind", when they heard that these two armies were approaching their capital. Isaiah tells Ahaz that however intimidated he may be, God's command is not to make this alliance with Syria and Israel. Instead, Ahaz is to stand firm in his faith and trust that God will deliver him. In fact, God even offers Ahaz a sign through the prophet Isaiah as assurance of what God has promised. Ahaz refuses the sign claiming that he doesn't wish to test God but Isaiah says that he will be given a sign anyway. That sign is the child that is promised in Isaiah 7:14. Isaiah says that this child will be born and before he is old enough to understand right from wrong the two nations which Ahaz fears will be laid waste. This child is given the name Immanuel because his birth was a sign that God (el) was with the people of Judah (Immanu meaning "with us").
Of course, Matthew was not unaware of all of this when he decided to quote Isaiah 7:14 in his writing of the gospel. He knew that the child of which Isaiah spoke was one that would be born in Isaiah's day and not his own and Matthew was not trying to negate that original meaning. Nor do I think Matthew was simply looking for any Old Testament prophecy about the birth of a child which he could then use to show that Jesus' birth was really predicted hundreds of year earlier, regardless of the original context of that verse.
Instead, by quoting this verse, I believe Matthew wants us to see the story of Isaiah and Ahaz in the story of Jesus. Matthew's quoting of Isaiah 7:14 is not about prediction or proving that Jesus is the Messiah. It is Matthew's way of saying this story is like that story; this story I am telling about Jesus is like that story of God delivering Judah from its enemies, it is a story of "God with us". In Isaiah, the birth of Immanuel served as a sign, a real physical reminder of God's promise to deliver his people if they would trust in Him. In Matthew's gospel, he wishes to show us that the birth of Jesus is a sign, a real physical reminder of God's promise to deliver his people if they would trust in Him.
Of course, Matthew will go on to show throughout the rest of his gospel that he means the quoting of this verse from Isaiah in ways even more radical than this. Matthew reveals to us that Jesus is not merely a sign of God with us as Immanuel was only a sign in Isaiah but that Jesus himself is in fact God with us, the very presence of God himself in human flesh. As God was with the people of Judah to deliver them from their enemies, so God became even more present among us by taking on our own flesh in order to deliver us from our worst enemies; sin and death.