Vote To Go To War In Iraq

I have been wondering this, but have not chosen to look it up until now.

How can Barack Obama say he voted against the war in Iraq when he was not even in the Senate when the vote was taken? He was elected to the Senate in 2004 - two years after the war started.

Here is the link to the roll call vote - U.S. Senate Roll Call Votes 107th Congress - 2nd Session