In the nature of Microsoft product updates, Window 7 is entering its middle years. Generally, Microsoft comes out with a new update to Windows every three years and Windows 7 is currently 18 months into its game. In that time Microsoft has sold 350 licenses to the operating system making it perhaps the fastest selling software of all time. In a blog post on the 18-month mark of Windows 7, Microsoft says that nearly 90% of businesses are currently in Windows 7 migrations and that it is saving enterprises $140 per PC per year, a $131 return on investment.