Help with Javascript form script

[pphfl]

I have the form below and I am wanting some help or script suggestions to create a username and password check. If someone enters the wrong username and/or password I would like a message to appear alerting of the wrong entry. All info is stored in the server the form posts to.

<form action="http://domain.net/clientportal/default_login.asp" method="post" name="form1" onsubmit="return validatePasswords()"><table class="mceVisualAid" style="height: 152px;" align="center" border="0" width="170">
                    <tbody>
                        <tr>
                            <td class="mceVisualAid">
                            <h5>Username:</h5>
                            </td>
                            <td class="mceVisualAid">
                            <h1><input size="17" id="username" class="bodytext" name="username" type="text"></h1>
                            </td>
                        </tr>
                        <tr>
                            <td class="mceVisualAid" colspan="2">&nbsp;</td>
                        </tr>
                        <tr>
                            <td class="mceVisualAid">
                            <h5>Password:</h5>
                            </td>
                            <td class="mceVisualAid">
                            <h1><input size="17" id="password" lass="bodytext" name="password" type="text"></h1>
                            </td>
                        </tr>
                        <tr>
                            <td class="mceVisualAid" colspan="2">&nbsp;</td>
                        </tr>
                        <tr>
                            <td class="mceVisualAid">&nbsp;</td>
                            <td class="mceVisualAid">
                            <div align="right"><input value="http://www.domain.net/index.php/client-login" name="redirecturl" type="hidden"> <input src="http://domain.net/images/signin-2.png" mce_src="images/signin-2.png" value="Submit" name="Submit" type="image"></div>
                            </td>
                        </tr>
                    </tbody>
                </table>
            </form>