Someone

$result=db_query("select nid,url from {msm_site}");
 while ($sitelist = db_fetch_object($result)) {
    $options[$sitelist->nid] = $sitelist->url;//set options array
//Logic to precheck checkboxes
   $precheck=db_query("select uid,nid from msm_node_user where uid=%d and nid=%d",$me,$sitelist->nid);
    if ($check = db_fetch_object($precheck) && $check > 0){
          $campaignlist[$sitelist->nid] = 1;
}
else{
 $campaignlist[$sitelist->nid] = 0;
}

//End logic


  }


 $form['selectsites'] = array('#title' => 'Select sites that user should have access to','#type' => 'checkboxes',
'#options' => $options,
'#attributes' => array('checked' => $campaignlist)
);