Windows users pay an average $21.50 to line the pockets of third-party software developers such as Sun Microsystems, Novell and Alcatel-Lucent, according to a calculation by the Software Freedom Law Center (SFLC). 

The money covers legal settlements that Microsoft has agreed with third-party software developers for alleged patent infringements over the past three years. The SFLC denounced the costs as a "patent tax" in a posting on its website. 

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 are not just paying for the programmers who put the program together and the corporate operations that brought it to market," the group stated.

"You are also paying a hidden tax of well over $20 that Microsoft has had to pay to other patent holders."

The SFLC 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 $4bn in patent settlements that Microsoft has signed in the past three years.

Settlements over that period include a $1.25bn deal with Sun Microsystems, a $536m deal with Novell and most recently a $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 $100m each year to protect itself in 35 to 40 patent lawsuits each year.

Based on estimated sales of 200 million Windows XP copies over the same period, the average legal costs per Windows XP copy amounts to $21.50.

Although 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 on which the Linux kernel is potentially infringing. Microsoft executives have also publicly stated that they believe that Linux is infringing on its intellectual property.

Several vendors over the past years, including IBM and Nokia, have issued Linux patent pledges, promising not to sue developers or users of the operating system for any infringements.