Authenticate Active Directory Error

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VB

Dear Friends

I want to check whether entered username password exist in active directory
my Server is windows 2003
i m using Visual Studio 2010
my code is

Private Function ValidateActiveDirectoryLogin(ByVal Domain As String, ByVal Username As String, ByVal Password As String) As Boolean
        Dim Success As Boolean = False
        Dim Entry As New System.DirectoryServices.DirectoryEntry("LDAP://" & Domain, Username, Password)
        Dim Searcher As New System.DirectoryServices.DirectorySearcher(Entry)
        Searcher.SearchScope = DirectoryServices.SearchScope.OneLevel
        Try
            Dim Results As System.DirectoryServices.SearchResult = Searcher.FindOne
            Success = Not (Results Is Nothing)
        Catch
            Success = False
        End Try
        Return Success
    End Function

this function is getting error as
Server is not operational

Please help me friends because i m new in this concept

Thanks in Advance
Sanju