Need to modify this script

Hi all,

I have this script which is working fine..

// tab ads

if (document.getElementById){
document.write('<style type="text/css">')
document.write('.multiparts, #formnavigation{display:none;}')
document.write('</style>')
}

var curpart=0

function getElementbyClass(classname){
partscollect=new Array()

var inc=0
var alltags=document.all? document.all : document.getElementsByTagName("*")
for (i=0; i<alltags.length; i++){
if (alltags[i].className==classname)
partscollect[inc++]=alltags[i]
}
}

function cycleforward(){
partscollect[curpart].style.display="none"
curpart=(curpart<partscollect.length-1)? curpart+1 : 0
partscollect[curpart].style.display="block"
//updatenav()
}

function cycleback(){
partscollect[curpart].style.display="none"
curpart=(curpart>0)? curpart-1 : partscollect.length-1
partscollect[curpart].style.display="block"
//updatenav()
}


function updatenav(){
document.getElementById("backbutton").style.visibility=(curpart==0)? "hidden" : "visible"
document.getElementById("forwardbutton").style.visibility=(curpart==partscollect.length-1)? "hidden" : "visible"
}

function onloadfunct(){
getElementbyClass("multiparts")
partscollect[0].style.display="block"
document.getElementById("formnavigation").style.display="block"
//updatenav()
}

if (window.addEventListener)
window.addEventListener("load", onloadfunct, false)
else if (window.attachEvent)
window.attachEvent("onload", onloadfunct)
else if (document.getElementById)
window.onload=onloadfunct

Code (markup):

Which goes forward with

<a id="forwardbutton" href="javascript:cycleforward()">

Code (markup):

Normally the images viewed by clicking the forward button.

My question is, How can I automate this like a slideshow ?
Can a piece of code be added in order to do it automaticly ?
Image will change every 3-5 seconds by it self , when clicked forward, it will go back to manual again.

Can anybody help ?