Spurred by the home buyer tax credit, and increased optimism in the economy, the housing market continued on Friday to show improvement. The Commerce Department said that sales of new homes rose in March to their highest levels since last summer. Overall, the sales of new single family houses in March were up nearly 27 percent at a seasonally adjusted annual rate of 411,000 units, the Commerce Department reported. The increase was against a revised rate of 324,000 for February and it exceeded expectations.