GlobalMemoryStatusEx

// Use to convert bytes to KB
#define DIV 1024

// Specify the width of the field in which to print the numbers. 
// The asterisk in the format specifier "%*I64d" takes an integer 
// argument and uses it to pad and right justify the number.
#define WIDTH 7

void _tmain()
{
  MEMORYSTATUSEX statex;

  statex.dwLength = sizeof (statex);

  GlobalMemoryStatusEx (&statex);

  _tprintf (TEXT("There is  %*ld percent of memory in use.\n"),
            WIDTH, statex.dwMemoryLoad);
  _tprintf (TEXT("There are %*I64d total KB of physical memory.\n"),
            WIDTH, statex.ullTotalPhys/DIV);
  _tprintf (TEXT("There are %*I64d free  KB of physical memory.\n"),
            WIDTH, statex.ullAvailPhys/DIV);
  _tprintf (TEXT("There are %*I64d total KB of paging file.\n"),
            WIDTH, statex.ullTotalPageFile/DIV);
  _tprintf (TEXT("There are %*I64d free  KB of paging file.\n"),
            WIDTH, statex.ullAvailPageFile/DIV);
  _tprintf (TEXT("There are %*I64d total KB of virtual memory.\n"),
            WIDTH, statex.ullTotalVirtual/DIV);
  _tprintf (TEXT("There are %*I64d free  KB of virtual memory.\n"),
            WIDTH, statex.ullAvailVirtual/DIV);

  // Show the amount of extended memory available.

  _tprintf (TEXT("There are %*I64d free  KB of extended memory.\n"),
            WIDTH, statex.ullAvailExtendedVirtual/DIV);
}

http://msdn.microsoft.com/en-us/library/windows/desktop/aa366589(v=vs.85).aspx