In 2004, there were 215 million voting age adults in the United States.
197 million of them were citizens.
142 million were registered to vote in the presidential election.
125 million voted.
That means 17 million did not vote who were registered, and a total of 72 million did not (or could not) vote.
Source: U.S. Census Bureau, Current Population Survey, November 1996, 2000, and 2004.