Invading privacy: Microsoft uses software with backward names like Windows Genuine Advantage to inspect the contents of users' hard drives. The licensing agreement users are required to accept before using Windows warns that Microsoft claims the right to do this without warning.
Monopoly behavior: Nearly every computer purchased has Windows pre-installed -- but not by choice. Microsoft dictates requirements to hardware vendors, who will not offer PCs without Windows installed on them, despite many people asking for them. Even computers available with other operating systems like GNU/Linux pre-installed often had Windows on them first.
Windows has a long history of security vulnerabilities, enabling the spread of viruses and allowing remote users to take over people's computers for use in spam-sending botnets. Because the software is secret, all users are dependent on Microsoft to fix these problems -- but Microsoft has its own security interests at heart, not those of its users.
In 2005, a vulnerability was discovered that affects all versions of Windows from Windows 3.0, released in 1990 until Windows Server 2003 R2 from December 2005, with XP and later versions most severely affected. The problem, which affects the Windows Metafile image format, a format commonly used for clip-art and other vector images. Files containing specially crafted 'Escape codes' allow for arbitrary user-defined function code to be run when displaying the image fails.