In Windows, why should I avoid running my computer as an administrator?
Running your Windows computer (including 2000, XP, and Vista) as an administrator (or Power User) leaves your computer vulnerable to security risks and exploits, such as Trojan horses. Simply visiting an unfamiliar Internet site as an administrator can cause extreme damage to your computer, such as reformatting your hard drive, deleting all your files, and creating a new user account with administrative access.
To avoid these problems refrain from running your computer with administrative rights and instead practice the principle of least privilege (though you will still need an administrator account on your computer in order to perform system maintenance. For more, search the Microsoft knowledge base for article 825069.
You can search Microsoft's knowledge base at: http://support.microsoft.com/default.aspx
To perform tasks that require administrative privileges, you can temporarily log in as an administrator.
This document was adapted from the Microsoft article Why you should not run your computer as an administrator.