Username Changes - THINKPADfirstname

Here's my system - Thinkpad T41P running Win XP Pro SP2. Here's my problem. I do not seem to be able to change a username. I can go into Control Panel and the username looks right, my first name, followed by a space, followed by my last name. But when I tried to connect to the c: drive on this machine from another computer on my network, for example from my MAC, I get asked for a username and password and then get told that I do not have permission to access this server. I can try connecting as another user, for example my wife's account, which is set up the same way, first name, space and last name. That connect's fine. So I narrowed the problem down to my username. When I went into the Properties dialogue box for the c: drive and the sharing tab and then checked the permissions, I see my name and then in parentheses THINKPADfirstname. IIf I click Add