Giving PC owners an incentive to put Windows 8 on their machines helps Microsoft because, if they upgrade, they will be less likely to switch to a competing platform such as Apple's Mac OS.

However, because Windows 8's main innovation is its Metro interface, designed with touchscreens in mind, consumers who upgrade now could be disappointed if their machine can't take advantage of the new functionality.

In the past month, Microsoft has announced two offers for consumers to upgrade existing PCs to the new OS, which is expected to ship before the end of the year.

"The low-cost upgrade to Windows 8 is a wise strategy for Microsoft. The cost of upgrading is the main reason people stay on outdated versions of Windows," said David Johnson, a Forrester Research analyst.

The offers show that Microsoft is finally beginning to understand the strategic cost of having huge numbers of consumers on old versions of Windows, he said via email. Windows XP, for example, first shipped in 2001.