Following a ruling in 2002 Microsoft must comply by ensuring that users can use 3rd party products like internet browsers or media players. The DOJ tested XPSP2 with 3rd party software through 100s of tests ensuring the software behaved correctly. The DOJ alerted Microsoft to the fact that it may not be in compliance with the judgement and the government is assessing Microsoft's responses.
The US government is therefore seeking to work with Microsoft to make sure that no antitrust issues are violated, and that the Longhorn operating system (currently due in 2006?) will allow vendors to develop their own products that work well with the new OS.