How can I permanently change the Command Prompt in Windows 7?

At the command prompt type:

setx Prompt $p$g$_$f

Then reopen the command prompt.

For anyone that doesn't know how to get help for DOS/CMD commands, type:

setx /?

To get:

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

Create a new environment variable called PROMPT. Set the variable value to the desired prompt.

To change the appearance of your DOS Prompt (or "Command Prompt"), right-click on "Computer" (usually on your Windows Desktop or in the Windows Start Menu), then follow these steps:

1. Click the "Advanced system settings" label
2. Click the "Environment Variables..." button
3. Click the "New..." button (a "User variable" only effects you, and "System variables" effect all users)
4. Create a variable called "PROMPT" and insert your desired prompt text (as I've included for you in the final picture below)
5. Click the "OK" button to close the "Environment Variables" window
6. Click the "OK" button to close the "System Properties" window

This change will also survive reboots.