This event can occur when the user credentials have been stored using the "Stored user names and passwords" applet in the control panel. It is possible to store credentials for automatic use (on XP and Server 2003) when connecting to network resources. When in place, any drive mapping or browsing attempt will automatically use any relevant stored credentials, even if the password for those credentials is no longer valid. In my case, it eventually locked out the stored user's account. See the link to "Stored User Names and Passwords" for some info on stored credentials.
This event is also recorded when FrontPage is used to connect to a website with a different account from the one currently logged in.
As per Microsoft: "A user who is logged on tried to create another logon session with a different user's credentials. Typically, this occurs when the user runs the RUNAS command and specifies a different set of credentials". See MSW2KDB
for more details.