User account control (or UAC) was introduced as another security tool in Windows Vista. With this feature enabled, any program that runs on the computer doesn't get administrator privileges by default. Operating system prompts user before running any application, thus preventing malware from being able to compromise your security.

While being effective in some cases, this feature can also be extremely annoying.

There are 3 different ways to enable or disable UAC.

1. Registry editor.

Go to Start->Run, type regedit and search for the following key:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\System

Locate EnableLUA key and set its value to 0.

Close Registry editor.

2. System configuration utility.

Go to Start->Run, type msconfig. Go to Tools tab and locate "Enable UAC" or "Disable UAC" line .

Note: it's really called UAP in some versions. 

3. Using User Accounts control panel applet.

1. Go to Start->Control Panel->User accounts.
2. Click Add or Remove user accounts: 
3. 
4. Click Change Security Settings
5. 
6. Check or uncheck "Use user account control to protect your computer"
7.  
8. 

Also, in some versions of Vista, you may skip steps 2-6, as "Enable or disable user accounts control" is right under User accounts.