function DisableTextSelection()
{
//Prevent selecting text in IE
document.onselectstart = new function(){return "false"};

//Prevent selecting text in FF
if (window.sidebar)
{
document.onmousedown = DisableText;
document.onclick = EnableText;
}
}

function DisableText(e)
{
return false;
}

function EnableText()
{
return true;
}

Now, whenever you want to disable text selection, just call DisableTextSelection();.