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By default the guest account in Windows 8 is turned off.
To activate it is much the same as Windows 7, just getting there is a little different.
Open the Control Panel by typing Control on a blank part of the Start screen and pressing Enter,
In Category view click on Add or remove user accounts.
You will see the Guest account and the fact that it is turned off, double click on it.
Click on 'Turn On' and its activated, close the Control panel and log off or reboot to test your Guest account if you wish.
The next time you Start your machine or logoff you will be given the option to log onto the Guest account or your normal account.
To turn it off again log in with your account. Navigate back to the User accounts as per above. Click on the 'Guest' account image and you will be given the option to turn it off.