From the BBC — the fine print in the new version of Windows grants Microsoft the right to delete whatever it wants to from your computer, and there’s nothing you can do about it:
Vista also incorporates Windows Defender, a security 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… even though that may result in other software ceasing to work or mistakenly result in the removal of software that is not unwanted…
For those users frustrated by the software’s limitations, Microsoft cautions that "you may not work around any technical limitations in the software"…
Sweet deal. (Disclosure, umpteenth time: I’m an Apple shareholder.)