<form id="form1" runat="server">
    <div>
    <asp:TextBox ID="txtSearchMask" runat="server"></asp:TextBox>
    <asp:Button ID="btnSearchAD" runat="server" Text="Search" OnClick="SearchAD" />

    <asp:ListBox ID="lstSearchResults" runat="server"></asp:ListBox>
    </div>
    </form>

Protected Sub SearchAD()
        lstSearchResults.Items.Clear()
        Dim myDE As New DirectoryEntry("LDAP://local.domen.com", "nachalnik", "123")
        Dim dirSearcher As DirectorySearcher = New DirectorySearcher(myDE)
        dirSearcher.Filter = "(&(objectCategory=Person)(objectClass=user)(SAMAccountName=*" & txtSearchMask.Text & "*))"
        dirSearcher.PropertiesToLoad.Add("Name")
        Dim results As SearchResultCollection = dirSearcher.FindAll()
        For Each result In results
            Dim myUser As New DirectoryEntry(result.Path.ToString())
            lstSearchResults.Items.Add(myUser.Properties.Item("name").Value.ToString())
        Next
    End Sub