If you correctly markup your HTML code, there is no need for javascript. The following code will allow the user to click on the label text to tick the checkbox.
<label for="surname">Surname</label>
<input type="checkbox" name="surname" id="surname" />
The for attribute on the label element links to the id attribute on the input element and the browser does the rest.
This has been testing to work in:
- IE6
- IE7
- Firefox