Can you set the id on the textbox? Then you can use getElementByID("textboxid"); Selecting by id is faster.
UPDATE:
You need to ensure that the DOM has been load so that your script can locate the element you want to update. One way is:
<body>
<form>
<input id="txtName" name="txtaaaName" type="text" value="notnow"/>
</form>
<script type="text/javascript">
function LoadTextBox(){
document.getElementById("txtName").value = "ready to go";
}
LoadTextBox();
</script>
</body>
An alternative could be where you use onload in the body tag:
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Untitled Document</title>
<script type="text/javascript">
function LoadTextBox(){
document.getElementById("txtName").value = "ready to go";
}
</script>
</head>
<body onload="LoadTextBox">
<form>
<input id="txtName" name="txtaaaName" type="text" value="notnow"/>
</form>
</body>