How can I get all Users from one Domain in a ListBox

You need to use the NetUserEnum api call, i have attached a function and the relevate declarations required for it, all you need to do is input the servername and the listbox where you want the usernames. You will have to get a list of servers (machines) in the domain before calling this function, if you need help with that just ask.

'Module Code

Public Const FILTER_NORMAL_ACCOUNT = &H2

Public Type MungeLong
X As Long
Dummy As Integer
End Type

Public Type MungeInt
XLo As Integer
XHi As Integer
Dummy As Integer
End Type

Public Declare Function NetUserEnum0 Lib "NETAPI32.DLL" Alias "NetUserEnum" (ServerName As Byte, ByVal Level As Long, ByVal lFilter As Long, Buffer As Long, ByVal PrefMaxLen As Long, EntriesRead As Long, TotalEntries As Long, ResumeHandle As Long) As Long
Public Declare Function PtrToInt Lib "kernel32" Alias "lstrcpynW" (RetVal As Any, ByVal Ptr As Long, ByVal nCharCount As Long) As Long
Public Declare Function PtrToStr Lib "kernel32" Alias "lstrcpyW" (RetVal As Byte, ByVal Ptr As Long) As Long
Public Declare Function StrLen Lib "kernel32" Alias "lstrlenW" (ByVal Ptr As Long) As Long
Public Declare Function NetAPIBufferFree Lib "NETAPI32.DLL" Alias "NetApiBufferFree" (ByVal Ptr As Long) As Long

Public Function EnumerateUsers(ByVal SName As String, lstCtrl As ListBox) As Long

Dim Result As Long, BufPtr, EntriesRead, TotalEntries As Long
Dim ResumeHandle, BufLen As Long
Dim SNArray(), GNArray(), UNArray(99) As Byte
Dim Uname As String, I As Integer, UNPtr As Long, TempPtr As MungeLong, TempStr As MungeInt

SNArray = SName & vbNullChar
GNArray = GName & vbNullChar

BufLen = 255
ResumeHandle = 0

'Enumerate all users on the given server
Result = NetUserEnum0(SNArray(0), 0, FILTER_NORMAL_ACCOUNT, BufPtr, BufLen, EntriesRead, TotalEntries, ResumeHandle)

'Set the return of this function to the return of the API call function
EnumerateUsers = Result

'If an error occured during the api call exit the function
If Result <> 0 And Result <> 234 Then
Exit Function
End If

'Loop through all entries returned by the API call and retrieve the string of the username
For I = 1 To EntriesRead
Result = PtrToInt(TempStr.XLo, BufPtr + (I - 1) * 4, 2)
Result = PtrToInt(TempStr.XHi, BufPtr + (I - 1) * 4 + 2, 2)

LSet TempPtr = TempStr
Result = PtrToStr(UNArray(0), TempPtr.X)
Uname = Left(UNArray, StrLen(TempPtr.X))
UserNames(I) = Uname
Next I

Result = NetAPIBufferFree(BufPtr)

For a = 1 To EntriesRead
'Add entries to the specified listbox
lstCtrl.AddItem UserNames(a)
Next a

End Function

Hope this helps you