Users can also add Windows Media Center for free once they upgrade to Windows 8 Pro.
The promotion was shared Monday on the Microsoft Windows Team blog.
"We set out to make it as easy as possible for everyone to upgrade to Windows 8," the blog said. "Starting at general availability, if your PC is running Windows XP, Windows Vista, or Windows 7 you will qualify to download an upgrade to Windows 8 Pro for just $39.99 in 131 markets."
The cost is substantially less than the $69.99 price for a a packaged DVD version of the upgrade at a local store. Microsoft said the upgrade promotion for Windows 8 Pro both online and at retail runs through January 31, 2013.