While China's antitrust complaint against Microsoft may in some ways resemble decade-old charges in the US and Europe, it is different in one major way: speed. Just a month after confirming that it was investigating the software giant for antitrust violations in that country, China has given Microsoft 20 days to formally respond to the complaint.

In a short statement, China's State Administration for Industry and Commerce (SAIC) said that Microsoft now has 20 days to respond in writing to antitrust charges related to Windows and Office. It delivered the ultimatum to senior Microsoft executives in China, including vice president David Chen.

"Microsoft may have violated China's anti-monopoly law," the state-run Xinhua news agency reported Monday.

"We strictly adhere to the relevant laws and rules in China and we have been actively cooperating with the SAIC's investigation," a Microsoft statement says in response.

China's investigation of Microsoft was both sudden and unexpected, and the SAIC hasn't really explained publicly what it's really after. The investigation appears to have been instigated by complaints from other businesses in China, and Xinhua previously reported that some of these businesses claimed Microsoft "used tie-in sales and verification codes" with Windows and Office that may violate China's new antitrust laws. A SAIC official said separately that the agency was also investigating how Microsoft bundles its media player and web browser in Windows.

China is also racing to develop its own computer operating system, which it hopes to use to replace Windows and, eventually, the Android mobile OS. The China government is very suspicious of non-Chinese technology, especially that which can help its oppressed population learn about the outside world and its own human rights violations.

Microsoft CEO Satya Nadella will reportedly visit China later in September, though this trip was planned before China unleashed its antitrust regulators. So it's unclear whether he will discuss this matter with China.