A very good reason to do this is if someone else that uses the computer with you accidentally makes changes and messes things up.


[You can create separate accounts for each person that will be using the computer.
This allows each user to have their own document folders and settings such as the wallpaper, Start menu, visual style, and so forth. You can create and configure user accounts with the User Accounts tool in Control Panel.

To open the User Accounts tool, open Control Panel from the Start menu, and then double click User Accounts.]
from Microsoft more info -
http://support.microsoft.com/?kbid=279783

APPLIES TO
• Microsoft Windows XP Home Edition
• Microsoft Windows XP Professional]
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and another site with "how to"do it with screen shots
[Windows XP gives you the ability to create user profiles, which allows each user on your computer to have a separate log in. There are also additional benefits, such as allowing each user to have their own document folders and personal settings, such as desktop icons, Start menu items, favorites, etc.
This also allows for greater privacy and less hassle of changing individual preferences for each user.

Note: You must have a computer administrator account on the computer and be logged in as an administrator to add a new user to the computer. Make sure this is correct for your computer before continuing. ]
http://www.worldstart.com/tips/tips.php/2891