Auto tab without destination

hi
i need to have auto tab functionality in my page. dont know abt the destination as page is generation dynamicaly.i m trying with tab index property

This function provide auto tab functionality in the field
function AutoTab(field)
{

var maxlen = document.getElementById(field).maxLength;
var currlen = document.getElementById(field).value.length;
if( currlen == maxlen && window.event.keyCode != "8")
{
//document.getElementById(field).value = document.getElementById(field).value + String.fromCharCode(window.event.keyCode);
window.event.keyCode = "9";
}
}

thsi funtion check for max lentgh of control and automatically switch focus to next.
but problem i m facing with onkeypress event. not able to fire window.event.keyCode = "9"; for keypress...
please suggest

thanxs