U.S. antitrust enforcers have started to track Microsoft Corp.'s development work on Longhorn, the successor to Windows XP, in a move to ensure that the vendor won't violate the final judgment in the government's antitrust case.

The U.S. Department of Justice (DOJ) and several U.S. states involved in the case are particularly interested in any changes that Microsoft might make in Longhorn related to default settings and "middleware," or applications associated with the operating system, such as Web browser and media player software, according to a legal filing on Friday.

As part of the final judgment, Microsoft has to make it easy for users and PC vendors to remove access to Microsoft middleware products and select competing products instead. Additionally, Windows can't be programmed to automatically change the configuration of icons, shortcuts or menu entries installed by a PC vendor.

"Early attention to these issues will enable plaintiffs and Microsoft to address any potential concerns in a timely matter, before the final structure of the product is locked into place," the filing reads.