[Windows Phone 8 Development and Hacking] petard:
There is a file "setx.exe" in the Windows folder

SetX has three ways of working:

Syntax 1:
SETX [/S system [/U [domain\]user [/P [password]]]] var value [/M]

Syntax 2:
SETX [/S system [/U [domain\]user [/P [password]]]] var /K regpath [/M]

Syntax 3:
SETX [/S system [/U [domain\]user [/P [password]]]]
/F file {var {/A x,y | /R x,y string}[/M] | /X} [/D delimiters]

Description:
Creates or modifies environment variables in the user or system
environment. Can set variables based on arguments, regkeys or
file input.

Parameter List:
/S system Specifies the remote system to connect to.

/U [domain\]user Specifies the user context under which
the command should execute.

/P [password] Specifies the password for the given
user context. Prompts for input if omitted.

var Specifies the environment variable to set.

value Specifies a value to be assigned to the
environment variable.

/K regpath Specifies that the variable is set based
on information from a registry key.
Path should be specified in the format of
hive\key\...\value. For example,
HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Control\TimeZoneInformation\StandardName.

/F file Specifies the filename of the text file
to use.

/A x,y Specifies absolute file coordinates
(line X, item Y) as parameters to search
within the file.

/R x,y string Specifies relative file coordinates with
respect to "string" as the search parameters.

/M Specifies that the variable should be set in
the system wide (HKEY_LOCAL_MACHINE)
environment. The default is to set the
variable under the HKEY_CURRENT_USER
environment.

/X Displays file contents with x,y coordinates.

/D delimiters Specifies additional delimiters such as ","
or "\". The built-in delimiters are space,
tab, carriage return, and linefeed. Any
ASCII character can be used as an additional
delimiter. The maximum number of delimiters,
including the built-in delimiters, is 15.

/? Displays this help message.

NOTE: 1) SETX writes variables to the master environment in the registry.

2) On a local system, variables created or modified by this tool
will be available in future command windows but not in the
current CMD.exe command window.

3) On a remote system, variables created or modified by this tool
will be available at the next logon session.

4) The valid Registry Key data types are REG_DWORD, REG_EXPAND_SZ,
REG_SZ, REG_MULTI_SZ.

5) Supported hives: HKEY_LOCAL_MACHINE (HKLM),
HKEY_CURRENT_USER (HKCU).

6) Delimiters are case sensitive.

7) REG_DWORD values are extracted from the registry in decimal
format.

Examples:
SETX MACHINE COMPAQ
SETX MACHINE "COMPAQ COMPUTER" /M
SETX MYPATH "%PATH%"
SETX MYPATH ~PATH~
SETX /S system /U user /P password MACHINE COMPAQ
SETX /S system /U user /P password MYPATH ^%PATH^%
SETX TZONE /K HKEY_LOCAL_MACHINE\System\CurrentControlSet\
Control\TimeZoneInformation\StandardName
SETX BUILD /K "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows
NT\CurrentVersion\CurrentBuildNumber" /M
SETX /S system /U user /P password TZONE /K HKEY_LOCAL_MACHINE\
System\CurrentControlSet\Control\TimeZoneInformation\
StandardName
SETX /S system /U user /P password BUILD /K
"HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\
CurrentVersion\CurrentBuildNumber" /M
SETX /F ipconfig.out /X
SETX IPADDR /F ipconfig.out /A 5,11
SETX OCTET1 /F ipconfig.out /A 5,3 /D "#$*."
SETX IPGATEWAY /F ipconfig.out /R 0,7 Gateway
SETX /S system /U user /P password /F c:\ipconfig.out /X