I've often run into a situation where either a computer I use (or manage) has been set up with the wrong username,...
or someone else's machine has been set up with the wrong username. Perhaps it's a username that the user in question doesn't like, or maybe it's been misspelled or is just plain incorrect. The usual procedure is to rename the existing user account on that machine -- that is, if it's not a domain account -- but renaming an existing user account under Windows XP can be quirky.
When a user account is created, a number of resources are also created with it that carry the user's name. The most obvious is the user's profile directory on that machine, which is "branded" with the name; similarly branded objects exist in the Registry as well. If you change the user's name by hand, these objects will not be automatically renamed. If you go looking for a user directory for "Fred" and find instead a directory named "Phil," it's confusing, and you may wonder what happened to poor Fred!
Rather than renaming a user account that's been in use for some time, try this: Create a new user profile with the proper name, migrate the old profile into the new one and delete the old one after everything is settled. Copying a user's profile data in Windows XP can be done by right-clicking on My Computer | Properties, then Advanced | User Profiles | Settings and using the Copy To function to select a target profile folder.
Note that you cannot copy the profile for the currently logged-in user. You should also create the target user and log in as that user before copying in the old profile data.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators. He is also the author of the book Windows Server Undocumented Solutions.
More information on user account names: