This is the latest in many legal challenges for Bill Gates' Microsoft

It is concerned that the deal could give Microsoft too dominant a position within digital rights management.

The Commission said it expects to reach a decision by 5 January next year.

Microsoft is already appealing against a 497m euros ($604m; �333m) fine from the European Commission for anti-competitive behaviour.

US giants Microsoft and Time Warner agreed in April to buy out most of Xerox's stake in ContentGuard.

DVD protection

European regulators said the probe will involve a review of ContentGuard's integration into Microsoft's other product areas.

"The notified concentration may have spill-over effects on a number of related markets ranging from mobile telephony to word processors," the Commission said.

Microsoft spokesman Dirk Delmartino said the company understood that it was a "complex area".

Digital rights management or DRM technology makes it possible to prevent illegal copying of CDs, DVDs, computer games, software systems, and even sensitive electronic documents.