global class checkbox{
webservice static void check(Id localId) {
            Account acc = new Account();
            system.debug(localId);
            system.debug('localId');
            acc.Id = localId ;
            acc.Validated__c = true;
            update acc;
}
}

{!REQUIRESCRIPT("/soap/ajax/30.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/30.0/apex.js")}
 alert("This is it!");
if({!Account.Validated__c}=true)
{
sforce.apex.execute("checkbox","check",{localId:"{!Account.Id}"});
alert("This is it!");
}