
The monies cover legal settlements that Microsoft inked with third party software developers for alleged patent infringements over the past three year. The SFLC in a posting on its website denounced that costs as a "patent tax".
The open source advocacy group provides free legal services to open source developers and is an outspoken opponent of software patents.
"If you run a computer using Windows, you're not just paying for the programmers who put the program together and the corporate operations that brought it to market. You're also paying a hidden tax of well over US$20 that Microsoft has had to pay to other patent holders," the group stated.
It pointed out that Linux by comparison has never been found guilty of any patent infringements "making Linux a patent-tax-free alternative to Windows".
The figure is based on a total of US$4bn in legal patent settlements that Microsoft signed over the past three years. Settlements over that period include Sun Microsystems, and most recently a US$1.52bn payment to Alcatel-Lucent.
The legal costs of these patent deals further add to Microsoft patent costs. Microsoft has publicly stated that it spends about US$100m each year to protect itself in 35 to 40 patent lawsuits each year.
Based on estimated sales of 200m Windows XP copies over the same period, the average legal costs per Windows XP copy amounts to US$21.50.
Although to date there have been no patent infringement claims against Linux distributors or individual developers, few legal experts doubt that Linux is infringing on numerous patents.
A 2004 study claimed that it had identified 283 patents that the Linux kernel is potentially infringing upon. Microsoft executives also have publicly stated that they believe that Linux is infringing on its intellectual property.