This bit of javascript will check and uncheck all checkboxes in a group of checkboxes. The checkboxes are grouped by naming all the checkboxes by the same name.

Javascript Code:

function checkUncheckAll(checkAllState, cbGroup)
{
	// Check that the group has more than one element
	if(cbGroup.length > 0)
	{
		// Loop through the array
		for (i = 0; i < cbGroup.length; i++)
		{
			cbGroup[i].checked = checkAllState.checked;
		}
	}
	else
	{
		// Single element so not an array
		cbGroup.checked = checkAllState.checked;
	}
}

HTML Code:

<input type=checkbox name=checkall onclick="checkUncheckAll(this, grp1);">
<input type=checkbox name=grp1 id=bx1>
<input type=checkbox name=grp1 id=bx2>
<input type=checkbox name=grp1 id=bx3>
<input type=checkbox name=grp1 id=bx4>