Google has spoken to the European Commission about concerns it has over the way Microsoft's search tool is part of its Internet Explorer software.
Microsoft wants to improve its internet search tool
Microsoft has included a search box in its new version of Internet Explorer, called IE7, that automatically selects the firm's MSN search tool as default.
Google complains that this is biased and may direct traffic from its site.
While the complaint was informal, it underlines how Google and Microsoft view each other as their main rivals.
Microsoft argues that users can change the search box to default to any search engine they choose.
The companies have been steadily encroaching on each other's business and analysts said the tussle for supremacy is likely to define the direction of online businesses in coming years.
Google is offering an increasing number of free software products that perform many of the same functions as paid-for packages from companies including Microsoft.
Instead of selling the software to make a profit, Google generates earnings by selling advertising to firms that want access to the users of its free products.
Microsoft has identified this sort of software as one of the biggest threats to its business.
The software giant also has decided to concentrate more on its search products in an effort to lure search users from Google.
Google's comments to the EC came after Brussels sent a letter to Microsoft containing questions relevant to its new operating system Vista.
The EC has not launched an investigation into Vista or IE7 and Google's comments were informal.
The EC's look at Vista is entirely separate from its continuing legal dispute with Microsoft over Brussels' 2004 anti-competition ruling against the US giant.
Many of Microsoft's rival internet browser software, such as Mozilla's Firefox, are set up in a similar way and in fact direct users to Google even though they can ultimately choose their default provider.
Gary Schare, director of product management for Internet Explorer, said Microsoft's did not want to limit choice or steal market share from its rivals.
"This is designed to essentially keep the status quo," he said.
But Marissa Mayer, Google's vice-president of search products and user experience, said: "The market favours open choice for search, and companies should compete for users based on their quality of search services."
Analysts said that Google's comments may in fact have a negative effect.
"Once you start raising unfairness questions, and you're the dominant player, then it would be very easy for somebody to use those arguments against you," said industry analyst Rob Enderle.