acces à une variable d'environnement windows sous C++

d'ailleur la question est surtout pour ceux qui connaissent Windows; ça me dit pas le nom de la variable d'environnement (style PATH, TMP, TEMP,...) qui contient le chemin du répertoire systeme ou du répertoire du système d'exploitation (je me contenterai de l'un ou de l'autre)
 

STANDARDS CONFORMANCE  
      getenv(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,  
      POSIX.2, ANSI C

getenv(3C)                                                       getenv(3C)
 
 NAME
      getenv() - return value for environment name
 
 SYNOPSIS
      #include <stdlib.h>
 
      char *getenv(const char *name);
 
 DESCRIPTION
      getenv() searches the environment list (see environ(5)) for a string
      of the form name=value, and returns a pointer to the value in the
      current environment if such a string is present, otherwise a NULL
      pointer.  name can be either the desired name, null-terminated, or of
      the form name=value, in which case getenv() uses the portion to the
      left of the = as the search key.
 
 APPLICATION USAGE
      getenv() is thread-safe. It is not async-cancel-safe.
 
 WARNINGS
      getenv() returns a pointer to static data which can be overwritten by
      subsequent calls.
 
 EXTERNAL INFLUENCES
    Locale
      The LC_CTYPE category determines the interpretation of characters in
      name as single- and/or multi-byte characters.
 
    International Code Set Support
      Single- and multi-byte character code sets are supported.
 
 SEE ALSO
      exec(2), putenv(3C), environ(5).
 
 STANDARDS CONFORMANCE
      getenv(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,
      POSIX.2, ANSI C
 
 Hewlett-Packard Company            - 1 -  HP-UX Release 11.00: October 1997
 
Standard input: END