At the same time, the European Competition Commission served Microsoft with on millions of Windows 7 PCs in the EU over a year-and-a-half span ending last July.

The free pass over Windows RT, however, eliminates an investigation that would in many ways be a repeat of the 2009 exchange between the Commission and Microsoft.

"We have looked at Windows RT, and on the basis of our investigation so far, there are no grounds to pursue further investigation on this particular issue," Joaquin Almunia, the EU's head antitrust official, said at a news conference Wednesday. "But we will closely monitor all the elements of the Windows software and how Microsoft complies to [its] commitments."

Almunia's agency fielded complaints last summer that accused Microsoft of stymying other browser makers' efforts to build software that runs in Windows RT, the offshoot of Windows 8 designed for ARM-powered tablets.

Microsoft will launch its first-ever computing device, the tablet, on Friday.