European Commission officials are examining complaints lodged by rivals against Microsoft's Vista operating system, due to be launched by the end of this year.

"Several companies have expressed their concerns to the European Commission concerning Microsoft's Vista, but there are no formal complaints," said Jonathan Todd, a spokesman for the Commission's top antitrust official, Commissioner Neelie Kroes. He added: "The Commission is monitoring the situation, but there is no formal investigation".

Asked what the substance of the complaints were, Todd said that they have to do with bundling of products into Vista, but declined to be more specific.

In a similar case in the US, Department of Justice officials are looking at Vista's boot-up features and whether PC makers can customise the options presented to users when they switch on their PCs.

Todd also stressed that Microsoft should comply with the principles established by the Commission's ruling of March 2004, which found that the company had abused its dominant position in the PC operating system market, when "designing and implementing Vista".

On Wednesday Microsoft has to formally respond to a request from the Commission to explain why it believes it has complied with the 2004 ruling. If the Commission does not accept the company's arguments, it can demand Microsoft pay a daily fine of €2 million ($2.4 million).