Try my freeware tool "BuiltIn.exe"
http://www.optimumx.com/download/
C:\>BuiltIn.exe /?
BuiltIn Account Manager [Version 1.11]
Displays or manages the builtin Administrator or Guest account, without
knowing
what the current name is.
The syntax of this command is:
BuiltIn.exe [/details] [/newname:name] [/newpw
assword] [/disabled:y|n]
[/account:Guest] [\\computer] [/domain:name]
/details: Displays the account details, instead of just the name.
/newname: Allows you to rename the account, 20 characters max.
/newpw: Allows you to change the account password, 256 characters
max.
/disabled: Allows you to disable or enable the account. (If policy
allows)
/account: Specifies the Guest account instead. (Default is
Administrator)
\\computer Allows you to manage the account on a remote system.
/domain: Manages the domain account instead. (Overrides \\computer)
An argument of /? or -? displays this syntax and always returns 1.
A successful completion will return 0.