A connection between a computer and the Internet increases the level of risk from malicious software and external attackers, and as old risks are managed, new ones are discovered or created.

A significant factor that increases the risks from malicious software is the tendency to give users administrative rights on their client computers. When a user or administrator logs on with administrative rights, any programs that they run, such as browsers, e-mail clients, and instant messaging programs, also have administrative rights. If these programs activate malicious software, that malicious software can install itself, manipulate services such as antivirus programs, and even hide from the operating system.

Malicious software poses numerous threats to organizations, from intercepting a user's logon credentials with a keystroke logger to achieving complete control over a computer or an entire network by using a rootkit. Malicious software can cause Web sites to become inaccessible, destroy or corrupt data, and reformat hard disks.

A defense-in-depth strategy, with overlapping layers of security, is the best way to counter these threats, and the least-privileged user account (LUA) approach is an important part of that defensive strategy. The LUA approach ensures that users follow the principle of least privilege and always log on with limited user accounts. This strategy also aims to limit the use of administrative credentials to administrators, and then only for administrative tasks.

For more info see the new Applying the Principle of Least Privilege to User Accounts on Windows XP. http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/luawinxp.mspx#ELAA