Microsoft could rethink plans to phase out its Windows XP operating system if customers show they want to keep it but so far they have not, CEO Steve Ballmer said.

"XP will hit an end-of-life. We have announced one. If customer feedback varies we can always wake up smarter, but right now we have a plan for end-of-life for new XP shipments," Ballmer told a news conference on Thursday.

Microsoft (NASDAQ: MSFT) has announced that it will stop licensing Windows XP to computer makers and end retail sales by June 30.

Ballmer said most retailers sold computers with Vista, the latest version of its Windows operating system, and most consumers were choosing to buy Vista.

Some consumers have complained they were unable to buy XP at retail stores as consumers. They say that to get XP they must buy their computers as small businesses.

"In the business environment, we still have customers who are buying PCs with XP" because information technology departments often have to work with old machines, Ballmer said.

Vista requires high-speed central processing units installed only in newer machines.

Ballmer was also asked whether the company would appeal an 899 million euro ($1.42 billion) antitrust fine imposed by the Brussels-based European Commission in February.

"I really have nothing to say about that today, sorry," he said.

The company must decide by early May whether to appeal to the European Court of First Instance against the fine, imposed because the Commission found Microsoft had charged rivals high prices to discourage software competition.

The court upheld a 497 million euro fine and other antitrust penalties against Microsoft in a landmark decision in September.

Ballmer was speaking at a news conference called to announce the establishment of an "innovation center" in the Belgian city of Mons, near where rival Google (NASDAQ: GOOG) has a datacenter.

Asked whether Microsoft had decided to locate in Mons because Google was there, Ballmer said it had not.

He reiterated Microsoft's plan to go to Yahoo (NASDAQ: YHOO) shareholders if that company turned down its takeover offer of $43.6 billion."We've sent them a letter that says, 'it's a good price, please let us know. If you don't let us know, maybe your shareholders will think it's a good price,'" Ballmer said.