Since Windows Vista, Microsoft introduced the User Account Control (UAC). UAC is meant to help you prevent unauthorized changes to your computer. Whenever a task requires administrative rights for changing settings or installing software it prompts for your permission. By default, UAC is set to dim your desktop which makes all other parts of Windows inaccessible until you confirmed that dialogue, which requires a keyboard or mouse. For someone that is relying on eyegaze alone, it is difficult / impossible to confirm this dialogue and get out of it.
To avoid that an eye gaze user is locked in that dialogue, you can turn off the desktop dimming or even disable UAC completely.