In tip 2548 » The Windows 2000 Runas utility, I described the Runas utility.
Microsoft Knowledge Base Article 294676 contains the following summary:
In Windows 2000 and Windows XP you can run programs as a different user than the currently-logged on user. To do this in Windows 2000, the RunAs service must be running, and to do this in Windows XP, the Secondary Logon service must be running. The RunAs and Secondary Logon services are the same service with different names. This step-by-step article describes how to enable and use the Run As command on a Windows 2000 computer or a Windows XP computer.