Gracias JavierB por tu ayuda!!!!!!
Pero todavia no logro que funcione correctamente ya q me sigue haciendo lo mismo. Se envia cuando le doy aceptar a algun mensaje de error! 
A continuación pego todo el dodigo a ver si alguién puede encontrar el error! 
Muchas gracias!!! 
  
<HTML>
<HEAD>
<meta http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<TITLE>Hola</TITLE>
<script>
function check(){
    document.formul.enviar.value = "Procesando Formulario";
    document.formul.enviar.disabled = true;
    document.formul.submit();
}
var patron = new Array(2,2,4)
var patron2 = new Array(1,3,3,3,3)
function mascara(d,sep,pat,nums){
if(d.valant != d.value){
	val = d.value
	largo = val.length
	val = val.split(sep)
	val2 = ''
	for(r=0;r<val.length;r++){
		val2 += val[r]	
	}
	if(nums){
		for(z=0;z<val2.length;z++){
			if(isNaN(val2.charAt(z))){
				letra = new RegExp(val2.charAt(z),"g")
				val2 = val2.replace(letra,"")
			}
		}
	}
	val = ''
	val3 = new Array()
	for(s=0; s<pat.length; s++){
		val3[s] = val2.substring(0,pat[s])
		val2 = val2.substr(pat[s])
	}
	for(q=0;q<val3.length; q++){
		if(q ==0){
			val = val3[q]
		}
		else{
			if(val3[q] != ""){
				val += sep + val3[q]
				}
		}
	}
	d.value = val
	d.valant = val
	}
}
function Validar(form)
{
  if (form.Nombre.value == "")
  { alert("Por favor ingrese su nombre"); form.Nombre.focus(); return false; } 
  if (form.Email.value == "")
  { alert("Por favor ingrese su dirección de e-mail"); form.Email.focus(); return false; } 
  if (form.Domicilio.value == "")
  { alert("Por favor ingrese su domicilio"); form.Domicilio.focus(); return false; } 
  if (form.Telefono.value == "")
  { alert("Por favor ingrese su número de teléfono"); form.Telefono.focus(); return false; } 
  if (form.Email.value.indexOf('@', 0) == -1 ||
      form.Email.value.indexOf('.', 0) == -1)
  { alert("Dirección de e-mail inválida"); form.Email.focus(); return false; } 
  form.submit();
}
</script>
</HEAD>
<BODY>
<table width="400"  border="0" align="center" cellpadding="0" cellspacing="0">
  <tr>
    <td height="80" valign="top"><object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0" width="400" height="70">
      <param name="movie" value="cabezal_pop.swf">
      <param name="quality" value="high">
      <embed src="cabezal_pop.swf" quality="high" pluginspage="http://www.macromedia.com/go/getflashplayer" type="application/x-shockwave-flash" width="400" height="70"></embed>
    </object></td>
  </tr>
  <tr>
    <td><div align="center"> </div>      <table width="98%"  border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#003366" bgcolor="#F0FBFF">
      <tr>
          <td>
		  <form action='formulario.php' method='post' enctype="multipart/form-data" name='formulario' target='_self' id='formulario'  onsubmit="return check()" />
            <table width="89%"  border="0" align="center">
            <tr bgcolor="#003366">
              <td height="25" colspan="3" class="txt-bold style6"><div align="center">DATOS PERSONALES</div></td>
              </tr>
            <tr>
              <td width="105" class="style5">Nombre*</td>
              <td colspan="2" class="style5"><input name="Nombre" type="text" id="Nombre" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Apellido*</td>
              <td colspan="2" class="style5"><input name="Apellido" type="text" id="Apellido" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Fecha de nacimiento*</td>
              <td width="72" class="style5"><input name="fecha" type="text" onkeyup="mascara(this,'/',patron,true)" size="12" maxlength="10" /></td>
              <td width="154" class="style5">dd/mm/aaaa</td>
              </tr>
            <tr>
              <td class="style5">Domicilio</td>
              <td colspan="2" class="style5"><input name="Domicilio" type="text" id="Domicilio" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Código Postal </td>
              <td colspan="2" class="style5"><input name="CP" type="text" id="CP" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Ciudad*</td>
              <td colspan="2" class="style5"><input name="Ciudad" type="text" id="Ciudad" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Pais*</td>
              <td colspan="2" class="style5"><input name="Pais" type="text" id="Pais" size="30"></td>
            </tr>
            <tr>
              <td class="style5">Teléfono*</td>
              <td colspan="2" class="style5"><input name="Telefono" type="text" id="Telefono" size="30"></td>
            </tr>
            <tr>
              <td class="style5">E-mail*</td>
              <td colspan="2" class="style5"><input name="Email" type="text" id="Email" size="30"></td>
            </tr>
            <tr>
              <td colspan="3" class="txt"> </td>
              </tr>
            <tr bgcolor="#003366">
              <td height="25" colspan="3" class="txt"> <div align="center"><strong class="style6">CURRICULUM VITAE</strong></div></td>
              </tr>
            <tr>
              <td colspan="3" class="style5"> (Adjunte el archivo con un tamaño menor a 260 kb. en formato  .PDF, .DOC, .RTF o .TXT) </td>
              </tr>
            <tr>
              <td colspan="3" class="txt"><span class="txt style6"><input type='file' name='archivo1' id='archivo1'></td>
              </tr>
            <tr>
              <td colspan="3" class="txt"> </td>
            </tr>
            <tr bgcolor="#003366" class="txt">
              <td height="25" colspan="3" class="txt"><div align="center"><span class="style6"><strong>COMENTARIOS</strong></span></div></td>
              </tr>
            <tr>
              <td colspan="3"><FONT 
      face=Arial size=-1>
                <TEXTAREA name=Comentarios cols=39 rows=5 id="Comentarios"></TEXTAREA>
              </FONT></td>
              </tr>
          </table>
              <table width="89%"  border="0" align="center" cellpadding="0" cellspacing="0">
                <tr>
                  <td width="64%" height="1"><img src="images/px_azul.gif" width="100%" height="1"></td>
                  <td width="26%"><div align="center">
                      <input name="boton" type="image" id="boton" src="images/ima_enviar.gif" width="119" border="0" onClick="Validar(this.form)">
                  </div></td>
                  <td width="10%"><img src="images/px_azul.gif" width="100%" height="1"></td>
                </tr>
              </table>
              <div align="center"></div>
        </form></td>
        </tr>
    </table>    </td>
  </tr>
  <tr>
    <td height="27" valign="bottom"><object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0" width="400" height="17">
        <param name="movie" value="pie_pop.swf">
        <param name="quality" value="high">
        <embed src="pie_pop.swf" quality="high" pluginspage="http://www.macromedia.com/go/getflashplayer" type="application/x-shockwave-flash" width="400" height="17"></embed>
</object></td>
  </tr></table>
</BODY>
</HTML>