check = []; //this is an array that stores all the true/false values for each checkbox

function checkBox(id,id2,id3,id4)
	{

	if(check[id] != true) //if a value is not true, use this rather than == false, 'cos the first time no value will be set and it will be undefined, not true or false
		{
		document.getElementById('imgCheck' + id).src = "gfx/checkbox_true.png"; //change the image
		document.getElementById('inputCheck' + id).value = "true"; //change the field value
		check[id] = true; //change the value for this checkbox in the array
		document.getElementById('imgCheck' + id2).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id2).value = "false";
		check[id2] = false;
		document.getElementById('imgCheck' + id3).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id3).value = "false";
		check[id3] = false;
		document.getElementById('imgCheck' + id4).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id4).value = "false";
		check[id4] = false;
		}
	else
		{
		document.getElementById('imgCheck' + id).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id).value = "false";
		check[id] = false;
		document.getElementById('imgCheck' + id2).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id2).value = "false";
		check[id2] = false;
		document.getElementById('imgCheck' + id3).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id3).value = "false";
		check[id3] = false;
		document.getElementById('imgCheck' + id4).src = "gfx/checkbox_false.png";
		document.getElementById('inputCheck' + id4).value = "false";
		check[id4] = false;
		}
		document.getElementById('tocheck').value = id;
	}