Both companies, which make Firefox and Opera, respectively, are involved in the , which was accused in January of "shielding" IE from competition by bundling the browser with Windows.

"Using the Windows Update channel to update Internet Explorer in any way that undermines user choices is a clear example of how Microsoft uses its monopoly position to damage competition in related products," said Mitchell Baker, the chairman of Mozilla, in an e-mail Thursday. Baker was traveling and not immediately available for an interview.

Hakon Wium Lie, Opera Software's chief technology officer, echoed Baker, citing his company's long-standing issue with Microsoft's distribution method for IE. "We're concerned both about the bundling of IE with Windows, and about Microsoft using Windows Update to reset user choice," said Lie.

Baker and Lie took exception to the process that Microsoft is using to offer IE8 to customers running the older IE6 and IE7 editions. When users receive an upgrade offer to IE8 via Windows Update, accept the offer and install the browser, then select the "Use express settings" option, IE8 becomes the default browser on the machine, even if a rival had been previously pegged as such.

Microsoft has . "Users continue to have complete control over IE8 settings and behavior throughout the first-run experience and ongoing use," argued Eric Hebenstreit, IE lead program manager, in an entry to a company blog last week. "For example, if IE is not the default browser in Windows, the option to change this setting is presented in a wizard that runs the first time IE8 is launched."