This problem can be solve using client side script.
Now Your Checkbox will look like
<pre lang="HTML">
<asp:checkbox id="chkSelectAllTo" runat="server" text="Select All" onclick="SelectAll(this)" xmlns:asp="#unknown" />
and to handle this 'SelectAll' event write following script.
<pre lang="HTML">
<script type='text/javascript'>
function SelectAll(chkBxObj)
{
var gridObj = document.getElementById('GridObjID');
for(var index = 0 ; index < gridObj.rows.length; index++)
{
var checkBoxObj = gridObj.rows[index].children[0].children[0];
checkBoxObj.checked = chkBxObj.checked;
}
return false
}
</script>