BRUSSELS (Reuters) - The European Commission is checking whether Microsoft Corp.'s system of collecting personal data from Internet users breaks privacy laws, compounding the software giant's antitrust probe headaches in Europe.

The European Union's executive arm announced its investigation into Microsoft's free .NET Passport service in a written response to a question from Erik Meijer, a Dutch member of the European Parliament.

"The Commission is...looking into this as a matter of priority, in concertation with national data protection authorities, as regards the system's compatibility (or not) with EU data protection law," European Commissioner for the Internal Market, Frits Bolkestein, wrote.

His reply, given on May 7, was seen by Reuters on Saturday.

Meijer had raised questions about .NET Passport service, which is designed to collect personal information from Internet users via an e-mail address or other sites while they are engaged online in a purchase, a game or a bank transaction.

He said that failure to register with .NET Passport results in exclusion from many sites' services and that de-subscribing is not possible.

The Commission is already investigating Microsoft for allegedly designing its Windows operating system to work better with its own server software than those of its rivals.

It is also concerned that the U.S. software giant has allegedly tied its Media Player software to its operating system.