SCOTT JAGOW: Software pirates are probably Microsoft's worst enemy. They sell counterfeit versions of Windows and other products online on sites like eBay. Today, Microsoft said it's going after 55 of these dealers worldwide. From our Innovations Desk at North Carolina Public Radio, Janet Babin reports.

JANET BABIN: This is Microsoft's largest effort to reign in counterfeit dealers. The company says the fake software is defective and could create security breaches on a user's computer.

UK technology writer Barry Fox says initially counterfeit software was probably good for Microsoft, because it helped make Windows a universal operating system. But now it costs the company billons of dollars.

BARRY FOX: "Now that the system is established as the de facto standard, Microsoft certainly doesn't want people selling counterfeit software anymore so they're getting very aggressive."

Microsoft reportedly partnered closely with eBay to shut down online auctions selling counterfeits.

According to the Miami Herald, the two companies intervened in about 50,000 sales a year that appear to be violating copyright laws.

Microsoft tested physical software copies bought on eBay. 39 percent were counterfeits.

I'm Janet Babin for Marketplace.