Check Checkbox on Intranet Page VBA

i have been trying to automate the checking of over 400 checkboxes on an internal page. i beleive i have located the code of one of the buttons but am unable to get this to work.

    <INPUT type="checkbox" name=PrincipalId class="padding-right: 15px;padding-top: 0px;padding- 
bottom: 0px"
value="585"
 title="JohnSmith"
onClick="document.getElementById('idSelectAll').checked=false;" >
<INPUT type="hidden"   name=PrincipalName
value="Smith%2C%20John%20%28Senior%20Manager%29" >
</td><td class="ms-vb2">