The latest milestone figure was announced at Microsoft's annual meeting with financial analysts. It's the most recent data available from the company since Vista hit the 40 million mark in May.
The first service pack for Windows Vista is rumoured to be delayed until 2009, despite Microsoft's previous claims that it won't be a massive, feature-laden release. The Windows Vista SP1 was previously expected in 2008.
In its first month on sale after its launch on 30 January, 20 million copies of Windows Vista were sold, but since then sales have slowed down.
During the analysts' meeting, Microsoft CEO Steve Ballmer indicated that a number of major organisations had renewed multi-year contracts with the company. Ballmer added that he believed growth must come not only from expanding Microsoft's core software business, but by succeeding in newer ventures such as online services and audio hardware.