Si, infatti il mio problema è che di Javascript non è che ne sappia poi molto. Di seguito posto il Javascript che ho utilizzato:

// doTooltip function
// Assembles content for tooltip and writes
// it to tipDiv
/////////////////////////////////////////////////
var t1,t2; // for setTimeouts
var tipOn = false; // check if over tooltip link
function doTooltip(evt,num) {
if (!tooltip) return;
if (t1) clearTimeout(t1); if (t2) clearTimeout(t2);
tipOn = true;
// set colors if included in messages array
if (messages[num][2]) var curBgColor = messages[num][2];
else curBgColor = tipBgColor;
if (messages[num][3]) var curFontColor = messages[num][3];
else curFontColor = tipFontColor;
if (ie4||ie5||ns5) {
var tip = startStr + messages[num][0] + midStr + '<span style="font-family:' + tipFontFamily + '; font-size:' + tipFontSize + '; color:' + curFontColor + ';">' + messages[num][1] + '</span>' + endStr;
tipcss.backgroundColor = curBgColor;
tooltip.innerHTML = tip;
}
if (!tipFollowMouse) positionTip(evt);
else t1=setTimeout("tipcss.visibility='visible'",100);
}

var mouseX, mouseY;
function trackMouse(evt) {
standardbody=(document.compatMode=="CSS1Compat")? document.documentElement : document.body //create reference to common "body" across doctypes
mouseX = (ns5)? evt.pageX: window.event.clientX + standardbody.scrollLeft;
mouseY = (ns5)? evt.pageY: window.event.clientY + standardbody.scrollTop;
if (tipOn) positionTip(evt);
}

/////////////////////////////////////////////////////////////
// positionTip function
// If tipFollowMouse set false, so trackMouse function
// not being used, get position of mouseover event.
// Calculations use mouseover event position,
// offset amounts and tooltip width to position
// tooltip within window.
/////////////////////////////////////////////////////////////
function positionTip(evt) {
if (!tipFollowMouse) {
mouseX = (ns5)? evt.pageX: window.event.clientX + standardbody.scrollLeft;
mouseY = (ns5)? evt.pageY: window.event.clientY + standardbody.scrollTop;
}
// tooltip width and height
var tpWd = (ie4||ie5)? tooltip.clientWidth: tooltip.offsetWidth;
var tpHt = (ie4||ie5)? tooltip.clientHeight: tooltip.offsetHeight;
// document area in view (subtract scrollbar width for ns)
var winWd = (ns5)? window.innerWidth-20+window.pageXOffset: standardbody.clientWidth+standardbody.scrollLeft;
var winHt = (ns5)? window.innerHeight-20+window.pageYOffset: standardbody.clientHeight+standardbody.scrollTop;
// check mouse position against tip and window dimensions
// and position the tooltip
if ((mouseX+offX+tpWd)>winWd)
tipcss.left = mouseX-(tpWd+offX)+"px";
else tipcss.left = mouseX+offX+"px";
if ((mouseY+offY+tpHt)>winHt)
tipcss.top = winHt-(tpHt+offY)+"px";
else tipcss.top = mouseY+offY+"px";
if (!tipFollowMouse) t1=setTimeout("tipcss.visibility='visible'",100);
}

function hideTip() {
if (!tooltip) return;
t2=setTimeout("tipcss.visibility='hidden'",100);
tipOn = false;
}

document.write('<div id="tipDiv" style="position:absolute; visibility:hidden; z-index:100"></div>')