Quick overview:
One of the major innovations in Windows Vista was the User Account Control. This warns the user of changes to the system, but offered little to no setting options under Vista, which meant that this useful feature quickly fell into disrepute.
Under Windows 7, the user account control has been significantly revised. You can now use the slider to set when the user should be notified. You can access the setting either via "Control Panel->User Accounts" or via "Start and entering UAC in the search field".
The slider can now be used to conveniently set a warning level; you can also deactivate user account control completely.
I personally recommend the second-lowest level, as I find it a good compromise between security and annoying warnings.
Sincerely,
Raimund Machacek

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