Renaming a Windows file

I have a computer that I have setup for a person and while installing Win XP I entered “Mark” as the name instead of “Michael”. I thought that I can just go into user accounts in the control panel later and change it. I went to go and do that and it did change the user name to Michael. Michael (the person) ran MYOB the other day and the name “Mark” appeared as the default user to log into MYOB. I looked in C:\Documents and Settings and found “Mark” I tried to rename the folder, but I got a message that said I cannot rename a windows file as it is needed to run the system. How do I get this changed. It’s just a pain for Michael to remove “Mark” from MYOB everytime