I fixed my answer, and thanks for the advice everyone!. This code now works...anyone can use it to count completed activities :

I took the two codes online from the website:
http://rjpalombo.com/2013/11/counting-open-activities-salesforce/
http://mysalesforcestuff.blogspot.ca/2015/04/counting-open-and-closed-activities-in.html

(I AM NOT TAKING CREDIT FOR THIS)

Tweeked the codes alittle bit ....added a field called 'Completed Activities' for following objectsl Accounts, Contacts, and Opportunities

Below is all the information you need:

1) Class
 

public class ActivityUtils {
    
    //config
    
    String fieldToUpdate = 'Completed_Activities__c'; //this field must be added to each object we're updating
       
    //state
    set<id> accountIds;
    set<id> contactIds;
    set<id> opportunityIds;
  
    
    public ActivityUtils(sObject[] records) {
        accountIds = new set<id>();
        contactIds = new set<id>();
        opportunityIds = new set<id>();
        captureWhatAndWhoIds(records);
        addAccountIdsFromRlatedObjects();
    }
    
    public void updateAccountActivityCount() {
        if(accountIds.size() == 0) return;
                 updateActivityHistory('Account','WhatId', getStringFromIdSet(accountIds));
        
        
    }
    public void updateContactActivityCount() {
        if(contactIds.size() == 0) return;
               updateActivityHistory('Contact','WhoId', getStringFromIdSet(contactIds));

    }
    public void updateOpportunityActivityCount() {
        if(opportunityIds.size() == 0) return;
              updateActivityHistory('Opportunity','WhatId', getStringFromIdSet(opportunityIds));

    }
      
     
    private void updateActivityHistory(String objToUpdate, String queryFld, String updateIds) {
        string strQuery = 'SELECT Id, (SELECT Id FROM ActivityHistories Where ActivityDate >= THIS_FISCAL_YEAR ) FROM ' + objToUpdate + ' WHERE Id IN (' + updateIds + ')';        
System.debug(strQuery);
        sObject[] sobjects = new list<sobject>();
        for(sObject so : database.query(strQuery)) {
            ActivityHistory[] oActivities = so.getSObjects('ActivityHistories');
            Integer closedActivityCount = oActivities == null ? 0 : oActivities.size();
            sObject obj = createObject(objToUpdate, so.Id);
            obj.put(fieldToUpdate, closedActivityCount);
            sobjects.add(obj);
            system.debug('ActivityHistoryCount: ' + closedActivityCount);
        }
        update sobjects;
    }
    
    private void captureWhatAndWhoIds(sObject[] objects) {
        for(sObject o : objects) {
            Id whatId = (Id)o.get('WhatId');
            Id whoId = (Id)o.get('WhoId');
            if(whatId != null) {
                String objectName = getObjectNameFromId(whatId);
                if(objectName == 'account') accountIds.add(whatId);
                if(objectName == 'opportunity') opportunityIds.add(whatId);
            }
            if(whoId != null) {
                String objectName = getObjectNameFromId(whoId);
                if(objectName == 'contact') contactIds.add(whoId);
              }
        }
    }
    
    private void addAccountIdsFromRlatedObjects() {
        for(Opportunity o : [SELECT AccountId FROM Opportunity WHERE Id IN :opportunityIds]) accountIds.add(o.AccountId);
        for(Contact c : [SELECT AccountId FROM Contact WHERE Id IN :contactIds]) accountIds.add(c.AccountId);
    }
    
    private String getObjectNameFromId(Id objId) {
        String preFix = String.valueOf(objId).left(3).toLowercase();
        if(prefix == '001') return 'account';
        if(prefix == '003') return 'contact';
        if(prefix == '006') return 'opportunity';
         return '';
    }
    
    private String getStringFromIdSet(set<id> idSet) {
        string idString = '';
        for(Id i : idSet) idString+= '\'' + i + '\',';
        return idString == '' ? idString : idString.left(idString.length()-1); //If idString contains some ids we want to ensure we strip out the last comma
    }
    
    //The main part of the method below was taken from //Taken from http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_dynamic_dml.htm
    //However we've modified this to accept an object id
    private sObject createObject(String typeName, Id objId) {
        Schema.SObjectType targetType = Schema.getGlobalDescribe().get(typeName);
        if (targetType == null) {
            // throw an exception
        }
        
        // Instantiate an sObject with the type passed in as an argument
        //  at run time.
        return targetType.newSObject(objId);
    }
    
}

Task Trigger:
trigger TaskTrigger on Task (after insert, after update, after delete, after undelete) {
 
    sObject[] triggerRecords;
    if(!trigger.isDelete) triggerRecords = trigger.new;
    else triggerRecords = trigger.old;
 
    //Update Completed Activity Count
    ActivityUtils au = new ActivityUtils(triggerRecords);
    au.updateAccountActivityCount();
    au.updateContactActivityCount();
    au.updateOpportunityActivityCount();
    
    
    }


Event Trigger
trigger EventTrigger on Event (after insert, after update, after delete, after undelete) {
 
    sObject[] triggerRecords;
    if(!trigger.isDelete) triggerRecords = trigger.new;
    else triggerRecords = trigger.old;
 
    //Update Open Activity Count
    ActivityUtils au = new ActivityUtils(triggerRecords);
    au.updateAccountActivityCount();
    au.updateContactActivityCount();
    au.updateOpportunityActivityCount();
 
}


TestClass - you can find above this post