GetDiskFreeSpace problem

[Daok]

I have a problem with this code :

    LPCTSTR lpDirectoryName=NULL;  //Current disk         
        PULARGE_INTEGER lpFreeBytesAvailable=0;
        PULARGE_INTEGER lpTotalNumberOfBytes=0;    
        PULARGE_INTEGER lpTotalNumberOfFreeBytes=0;

        cout << GetDiskFreeSpaceEx(lpDirectoryName, 
            lpFreeBytesAvailable,
            lpTotalNumberOfBytes,
            lpTotalNumberOfFreeBytes);
    iSize = (unsigned int)(lpTotalNumberOfBytes); 

Why iSize is always not the real free space that I have ? What do I do wrong ?

reference:
http://msdn.microsoft.com/library/de...dmref_8bso.asp