The least-privileged user account  (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.

Thought y'all might want to see this - fresh off the press today:

http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/luawinxp.mspx