I basically have the exact problem this guy asked: http://www.experts-exchange.com/Operating_Systems/WinXP/Q_21054627.html
I’ll just copy his text here since it’s the same problem:
*How do I change a user name that’s under “Documents and Settings”?
If I right click and select rename, windows does not allow that folder to be renamed, giving some “security” warning.
I’ve been to control panel , but that name does not show up under “users”.*
I need to do this for two computers here at the office. One is my boss’s, and the name is the guy who set up the computer for him, and it just kind of annoys him that it’s there, plus it’s cluttery. The other one, and the funny thing, is that before he realized that it was hard/impossible to change this, he (my boss) had set up a computer for one of the other women in the office, and as a joke set up her first/default/whatever user as “BUTTMUNCH” which as you can probably imagine, would be nice to change.
I haven’t looked at her system, but I think it’s the same as his… on his the user “Kent” (the one we don’t want) no longer shows up in the user accounts under control panel, so it’s either been renamed or deleted… but the “Kent” folder still exists under “Documents and Settings” and can’t be deleted or renamed. I assume it’s the same for the “Buttmunch” thing on the other computer…
Really we’d rather not have to reinstall Windows… please tell me there is another answer?