Security ID's or SID's are unique.  You cannot rename an account and keep the same credentials. The ADMINISTRATOR account cannot be renamed nor can the GUEST account be renamed. Sysprep is a tool that will regenerate a unique SID So if there is no data on there you can use sysprep and generalize to reset the machine and create a new Login.

Sysprep Command-Line Syntax: http://technet.microsoft.com/en-us/library/cc721973(WS.10).aspx
What is Sysprep?: http://technet.microsoft.com/en-us/library/cc721940(WS.10).aspx
How Sysprep Works: http://technet.microsoft.com/en-us/library/cc766514(WS.10).aspx