This is how to return the client computer name of a Windows Terminal Services session (running on Windows 2000) -

Code:
Public Declare Function WTSQuerySessionInformation Lib "wtsapi32.dll" Alias "WTSQuerySessionInformationA" (ByVal hServer As Long, ByVal SessionID As Long, ByVal InfoClass As Long, ByRef ppBuffer As Long, ByRef pBytesReturned As Long) As Long
Public Declare Sub WTSFreeMemory Lib "wtsapi32.dll" (ByVal pMemory As Long)
Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Long, ByVal Length As Long)
Public Const WTS_CURRENT_SERVER_HANDLE As Long = 0
Public Const WTS_CURRENT_SESSION As Long = -1
Public Const WTSClientName As Long = 10

Public Function GetWTSClientName() As String
'This returns the name of the Windows Terminal Services client name that
'this server session is connected to.
Dim strBuffer As String
Dim lpBuffer As Long
Dim RetVal As Long
Dim pBytes As Long

RetVal = WTSQuerySessionInformation(WTS_CURRENT_SERVER_HANDLE, WTS_CURRENT_SESSION, WTSClientName, lpBuffer, pBytes)
If RetVal Then
    GetWTSClientName = GetStringFromLP(lpBuffer)
    WTSFreeMemory lpBuffer
End If

End Function

Private Function GetStringFromLP(ByVal StrPtr As Long) As String
'Returns a string, given the pointer to it
Dim b As Byte
Dim TempStr As String
Dim BufferStr As String
Dim Done As Boolean

    Done = False
    Do
        CopyMemory ByVal VarPtr(b), ByVal StrPtr, 1
        If b = 0 Then
            Done = True
        Else
            TempStr = Chr$(b)
            BufferStr = BufferStr & TempStr
            StrPtr = StrPtr + 1
        End If
    Loop Until Done
    GetStringFromLP = BufferStr
        
End Function