Cross-thread operation not valid: Help me? (sr my english, i'm from Vietnamese)

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
       {
           string date="";
           Int64  m_totalSize=0;
             for (int i = 0; i < listVSearch.Items.Count; i++)
           {
               try
               {
                   m_totalSize=0;
                   HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(listVSearch.Items[i].SubItems[3].Text);
                    HttpWebResponse webResp = (HttpWebResponse)webReq.GetResponse();
                   DateTime today = DateTime.Now;
                   if (DateTime.Compare(today, webResp.LastModified) == 1)
                     date = webResp.LastModified.ToShortDateString();
                    m_totalSize += webResp.ContentLength;
                     webResp.Close();
               }
               catch (Exception) { }
                 listVSearch.Items[i].SubItems[2].Text=date.ToString();
                 listVSearch.Items[i].SubItems[1].Text = (FormatSizeDecimal(m_totalSize, 2)).ToString();
           }
       }

 private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
{
...
backgroundWorker1.RunWorkerAsync();
}