"Vista also incorporates Windows Defender, an anti-virus program that actively scans computers for "spyware, adware, and other potentially unwanted software." The agreement does not define any of these terms, leaving it to Microsoft to determine what constitutes unwanted software.
Once operational, the agreement warns that Windows Defender will, by default, automatically remove software rated "high" or "severe," even though that may result in other software ceasing to work or mistakenly result in the removal of software that is not unwanted."
Full article here, an interesting read