Problema en validez de campo

[AlisHosting]

Hola amigos, tengo un problema en un script colocado en un fichero ASP, el problema es que cuando no insecto un dato en el campo que requiere me sale el mensaje msgbox y pulso acepta pero en vez de volver al formulario me carga la pagina donde recoge los datos del formulario alta.asp.

funcion:

<script LANGUAGE="JavaScript">

function Validar(form)
{

if (form.User2.value.indexOf('@', 0) == -1 ||
form.User2.value.indexOf('.', 0) == -1)
{ alert("Dirección de e-mail inválida"); form.User2.focus(); return; }

form.submit();
}

</script>

Formulario:
<form METHOD="post" ACTION="Alta/alta.asp"> <table width="100%" height="266" border="0" cellpadding="0" cellspacing="0">
<!--DWLayoutTable-->
<tr>
<td width="254" background="Images/HOSTING_08.jpg"><table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td><div align="center"></div>
<div align="center" class="Estilo4">Aun no soy cliente.</div></td>
</tr>
</table></td>
</tr>
<tr>
<td height="47" background="Images/HOSTING_09.jpg"><table width="100%" border="0" cellpadding="0" cellspacing="0" class="Estilo7">
<tr>
<td width="4%">&nbsp;</td>
<td width="34%"><div align="left">E-mail:</div></td>
<td width="62%">
<div align="left">
<input name="User2" type="text" id="User2" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><div align="left">Contrase&ntilde;a:</div></td>
<td>
<div align="left">
<input name="Pass2" type="password" id="Pass2" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td width="34%"><div align="left">Repetir contrase&ntilde;a</div></td>
<td width="62%">
<div align="left">
<input name="Pass22" type="password" id="Pass22" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><div align="left">Pers. Contacto:</div></td>
<td>
<div align="left">
<input name="Ap2" type="text" id="Ap2" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><div align="left">Telefono:</div></td>
<td><div align="left">
<input name="Tel2" type="text" id="Tel2" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><div align="left">Fax:</div></td>
<td><div align="left">
<input name="Fax2" type="text" id="Fax2" size="20">
</div></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><div align="left">&iquest;Donde nos conociste? </div></td>
<td><div align="left">
<select name="select">
<option value="Google">Google</option>
<option value="Otros Buscadores">Otros Buscadores</option>
<option value="Banner">Banner</option>
<option value="otro sitio web">otro sitio web</option>
<option value="otros medios">otros medios</option>
</select>
</div></td>
</tr>
<tr>
<td height="15">&nbsp;</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
</tr>
</table></td>
</tr>
<tr>
<td height="58" valign="top" background="Images/HOSTING_10.jpg"><div align="center"><br>
<span class="Estilo2"><a href="javascript:history.back()"><img src="Images/anterior.gif" width="92" height="26" border="0"></a>
<input name="image22" type="image" onClick="Validar(this.form)" value="Enviar" src="Images/Siguiente.gif" width="92" height="26" border="0">
<br>
</span></div></td>
</tr>
<tr>
<td height="6"></td>
</tr>
</table>
</form>