The so-called "world refund day" was organised by fans of Linux, an alternative operating system that is freely available for downloading on the Internet.
"The message is not 'Windows sucks, I want my money back'. The message is 'I choose not to accept this licence and am therefore exercising my right to return the product for a refund'," the organisers explained.
Fans of Linux and other operating systems argue that Microsoft's own rules permit them to refuse the company's products, but in practice it is almost impossible to do so.
Geoffrey Bennett, an Australian, spent four months exchanging e-mails with Microsoft and Toshiba, the manufacturer of his personal computer, which like almost every other computer maker "bundles" Windows into the software package included on its machines. Mr Bennett refused to sign his user's licensing agreement with Microsoft and pointed to a clause in the fine print stating that he was entitled to a refund.
Microsoft said he had to seek his refund from Toshiba, and Toshiba said he had to seek his refund from Microsoft. After much to-ing and fro-ing, he was sent 110 Australian dollars (pounds 45) from Toshiba.
Protesters were due to converge to Microsoft's offices in Foster City, south of San Francisco, Orange County and Santa Monica in the Los Angeles area, and in New York City.