Microsoft has been criticized by users that its earlier operating systems are not secure. So Microsoft introduced a new featureUser Access Control (UAC) with Windows Vista to improve the security of your system. This ensures that accidental system changes are not performed without your consent. If any kind of important system change is taking place by your action then it will ask for your permission by showing a dialog box. 

If you are logged on as a standard user and not as an Administrator and you are installing a new application, making changes to registry or writing to important system locations then Windows System asks for administrator credentials to avoid unauthorized or accidental changes. 

However Microsoft recommends "do not turn off UAC for security reasons". Even if you want to disable then please ensure that you have proper security measures in place like updated antivirus.

In Windows 7 Microsoft made the UAC less annoying and more controllable by providing a slider to set the notification level and disable it.

This article tells how to disable UAC in Windows 7 Using Registry Editor.

It is advised to backup your registry before making any changes.

Open Registry Editor by typing “regedit” in Start Search.

Navigate to the following registry key:

HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionPoliciesSystem

GO to EnableLUA DWORD in Right pane.

Set its new value to 0.

Optional step to suppress UAC permission prompt dialog,

GO to  ConsentPromptBehaviorAdmin DWORD in Right pane.

Set its new value to 0

Exit from Registry Editor.

Restart your System to apply changes.

Enable UAC again, by changing the value of EnableLUA to 1.