How to run javascript on pageload event

I need to run dynamic javascript code into pageload event ..and i need some default values placed on Textboxes using this script i wrote this sample code but i am getting javascript error ..
for ex:-

protected void Page_Load(object sender, EventArgs e)
{
string script = string.Empty;
script += "<script language='javascript'>";
script += "document.getElementById('TextBox1').value='a2'";
script += "</";
script += "script>";
ClientScript.RegisterClientScriptBlock(this.GetType(), "Error", script);
}
"; script += "document.getElementById('TextBox1').value='a2'"; script += ""; ClientScript.RegisterClientScriptBloc ...