Tks Jim,

It worked fine! Here's the code:

' The caller

       Dim iu As New ImpersonateUser
       If iu.impersonateValidUser("testUser", "MyDomain", "test1234") Then
           'Insert your code that runs under the security context of a
specific user here.
           Dim iop As New InteropTest.Net.Hello
           Response.Write(iop.World)
           Marshal.ReleaseComObject(iop)
           iu.undoImpersonation()
       Else
           'Your impersonation failed. Therefore, include a fail-safe
mechanism here.
           Response.Write("Did you think that it would work?")
       End If

' The class

Imports System.Security.Principal
Imports System.Runtime.InteropServices

Public Class ImpersonateUser

   Dim LOGON32_LOGON_INTERACTIVE As Integer = 2
   Dim LOGON32_PROVIDER_DEFAULT As Integer = 0

   Dim impersonationContext As WindowsImpersonationContext

   Private Declare Function LogonUserA Lib "advapi32.dll" (ByVal
lpszUsername As String, _
                           ByVal lpszDomain As String, _
                           ByVal lpszPassword As String, _
                           ByVal dwLogonType As Integer, _
                           ByVal dwLogonProvider As Integer, _
                           ByRef phToken As IntPtr) As Integer

   Private Declare Auto Function DuplicateToken Lib "advapi32.dll" ( _
                           ByVal ExistingTokenHandle As IntPtr, _
                           ByVal ImpersonationLevel As Integer, _
                           ByRef DuplicateTokenHandle As IntPtr) As Integer

   Private Declare Auto Function RevertToSelf Lib "advapi32.dll" () As Long
   Private Declare Auto Function CloseHandle Lib "kernel32.dll" (ByVal
handle As IntPtr) As Long

   Public Function impersonateValidUser(ByVal userName As String, ByVal
domain As String, ByVal password As String) As Boolean

       Dim tempWindowsIdentity As WindowsIdentity
       Dim token As IntPtr = IntPtr.Zero
       Dim tokenDuplicate As IntPtr = IntPtr.Zero
       impersonateValidUser = False

       If RevertToSelf() Then
           If LogonUserA(userName, domain, password,
LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, token) <> 0 Then
               If DuplicateToken(token, 2, tokenDuplicate) <> 0 Then
                   tempWindowsIdentity = New
WindowsIdentity(tokenDuplicate)
                   impersonationContext = tempWindowsIdentity.Impersonate()
                   If Not impersonationContext Is Nothing Then
                       impersonateValidUser = True
                   End If
               End If
           End If
       End If
       If Not tokenDuplicate.Equals(IntPtr.Zero) Then
           CloseHandle(tokenDuplicate)
       End If
       If Not token.Equals(IntPtr.Zero) Then
           CloseHandle(token)
       End If
   End Function

   Public Sub undoImpersonation()
       impersonationContext.Undo()
   End Sub

End Class