Ya lo he conseguido... :D
es con esto...
<script language="JavaScript" type="text/JavaScript">document.forms[0].elements[0].focus()</script>
con el forms, seleccionas el formulario que quieres y, con elements, el elemento quieres.
Código PHP:
<form action="altaempresa.php" method="post" name="mayusculas">
<table>
<tr>
<td width="158"><div align="left"><strong> Nombre </strong></div></td>
<td width="268"><input onKeyUp="mayusculas.nombre_empresa.value=mayusculas.nombre_empresa.value.toUpperCase();" type="text" name="nombre_empresa" size="33" maxlength="35"></td>
<td width="140" colspan="2" rowspan="11">
</td>
</tr><tr>
<td><div align="left"><strong> Dirección </strong></div></td>
<td><input type="text" name="direccion" size="33" maxlength="40"></td>
</tr><tr>
<td><div align="left"><strong> Población </strong></div></td>
<td><input type="text" name="poblacion" size="33" maxlength="20"></td>
</tr><tr>
<td><div align="left"><strong> Provincia </strong></div></td>
<td><input type="text" name="provincia" size="33" maxlength="20"></td>
</tr><tr>
<td><div align="left"><strong> C.P. </strong></div></td>
<td><input type="text" name="cpostal" size="33" maxlength="20"></td>
</tr><tr>
<td><div align="left"><strong> N.I.F. </strong></div></td>
<td><input type="text" name="nif" size="33" maxlength="20"></td>
</tr><tr>
<td><div align="left"><strong> E-mail </strong></div></td>
<td><input type="text" name="email" size="33" maxlength="45"></td>
</tr><tr>
<td><div align="left"><strong> Web </strong></div></td>
<td><input type="text" name="web" size="33" maxlength="45"></td>
</tr><tr>
<td><div align="left"><strong> Persona/s de Contacto </strong></div></td>
<td><input type="text" name="contacto" size="33" maxlength="20"></td>
</tr><tr>
<td><div align="left"><strong> Teléfono </strong></div></td>
<td><input type="text" name="telefono" size="33" maxlength="45"></td>
</tr><tr>
<td><div align="left"><strong> Mantenimiento </strong></div></td>
<td><input type="text" name="mantenimiento" size="33" maxlength="45"></td>
</tr><tr>
<td><div align="left"><strong> Notas </strong></div></td>
<td colspan="3"><textarea name="notas" cols="50" rows="10"></textarea></td>
</tr>
</table>
<br>
<script language="JavaScript" type="text/JavaScript">document.forms[1].elements[0].focus()</script>
<input type="submit" value="Aceptar">
</form>