But Microsoft stands behind its decision to limit IE9 to users running Windows Vista and Windows 7.

"It was a very deliberate decision," said Ryan Gavin, senior director of IE, talking about the move to exclude XP users from IE9. "You simply can't build on something that is 10 years ago."

Gavin said that Microsoft "has no second thoughts" about its decision.

According to California-based Net Applications, one of several companies that regularly publishes browser usage data, IE lost nine-tenths of a percentage point of share in March, falling to 55.9%, another record low.

IE9, which debuted more than two weeks ago, accounted for 1% of all browsers, a five-tenths of a point jump over February.