Eight officials from Japan's Fair Trade Commission (FTC) raided the Japanese offices of Microsoft this morning as part of that country's investigation into possible anti-competitive practices by the software behemoth.

"The FTC officials are conducting an investigation regarding provisions in Microsoft's licenses with PC manufacturers," said Aki Araki, a Microsoft Japan spokeswoman. "We are confident that our practices are consistent with Japanese law."

A team from Microsoft is continuing to help the FTC officials with their investigations, Araki said.

Microsoft licenses its patented software – most crucially the Windows operating system – to PC manufacturers. Previous probes in the US and Europe have focused on whether Microsoft illegally maintains its monopoly in PC operating systems through restrictive licensing agreements with PC makers.

Microsoft has allegedly forced Japanese PC makers to accept a contractual clause stipulating that they can't bring a case to court even if the software giant's technologies are very similar to those developed by Japanese firms, the Kyodo news agency reported Thursday, citing sources close to the action.

Local media sources quoted unnamed officials as saying that the current investigation focuses on the licensing for Windows XP. In 1998, Microsoft was warned by the FTC to stop engaging in anti-competitive business practices, but the company escaped punishment.

Japan is the world's third-largest market for PCs after the US and China.