Ну-ну. Тогда:
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The SetEnvironmentVariable function sets the value of an environment variable for the current process.

BOOL SetEnvironmentVariable(

LPCTSTR lpName, // address of environment variable name
LPCTSTR lpValue // address of new value for variable
);


Parameters

lpName

Points to a null-terminated string that specifies the environment variable whose value is being set. The operating system creates the environment variable if it does not exist and lpValue is not NULL.

lpValue

Points to a null-terminated string containing the new value of the specified environment variable. If this parameter is NULL, the variable is deleted from the current process's environment.



Return Values

If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.

See Also

GetEnvironmentVariable

Может, в логон-скрипт вписать? Раз до перезапуска сохраняется...