Microsoft has initiated a wide-ranging crackdown on illegal sales of its software.

The company has filed 55 lawsuits in 11 countries targeting the distribution of counterfeit copies of its software on eBay and other auction sites. The action is Microsoft's largest enforcement campaign to date, according to the company.

The 55 filings include 15 in the US, 10 in Germany, 10 in The Netherlands, and five in the UK. The counterfeit software named in the lawsuits ranges from Office 97 and Windows 95 to beta versions of Vista and Office 2007.

One of the suits filed in Germany accuses a seller of distributing more than 2,000 copies of pirated Microsoft products.

Redmond said that most of the illegal sellers were uncovered after the counterfeit software was checked with Windows Genuine Advantage (WGA), an anti-piracy program built in to many Microsoft applications.

WGA was credited with helping to file suits against eight companies in September 2005.

Microsoft claimed that the suits are protecting its intellectual property and the safety of its customers' PCs.

"Counterfeit software is defective and dangerous because counterfeiters tamper with the genuine software code. This leaves the door open to identity theft and other serious security breaches," said Microsoft senior attorney Matt Lundy.

To support the claim, Microsoft pointed to an in-house study on counterfeit copies of Windows XP conducted by the company in July.

The study claimed that 34 per cent of the counterfeit disks would not install on a PC, and that 43 per cent contained additional code not included with commercial versions of Windows XP.

Microsoft also cited an IDC study the company sponsored earlier this month, suggesting that fewer than half of the Microsoft products available on eBay are legally licensed and not tampered with.