Spain's antitrust authorities have opened an investigation into Microsoft for allegedly blocking third parties from selling software licences.
The National Competition Commission (CNC) said on Tuesday that the investigation may take up to 18 months.
The CNC said a company called Elegant Business SC had complained that Microsoft was preventing or unreasonably restricting the resale of software licences, particularly PC operating systems licences. According to Elegant Business's complaint, this is in breach of Spanish competition law.
In a statement, the CNC said its investigators have already "had access to certain information which may point to the existence of prima facie evidence that Microsoft has breached articles 1 and 2 of the Spanish Competition Act and articles 101 and 102 of the Treaty on the Functioning of the European Union, as a consequence of Microsoft's actions and agreements aimed at unjustifiably preventing or limiting the resale of software licences of this operator".
Microsoft told ZDNet UK it had received a formal notification of investigation and would "work closely with the authorities on the issues in hand".
The company has had many run-ins with European competition authorities. In 2004, the European Commission fined the company €497m for not letting rivals compete fairly in the server software market. Microsoft said the fine was excessive, and its failure to pay led the Commission to keep piling on additional fines.
The Commission also forced Microsoft to give buyers of Windows PCs a choice at first start-up as to which browser they want to use, rather than just forcing them to start with Internet Explorer.
The CNC action opened up this week is levelled at Microsoft's subsidiaries in Spain and also in Ireland, where the company's European operations centre is based.