Try this fix-
attachment = New System.Net.Mail.Attachment(txtattach.Text)
if(attachment <> null)
e_mail.Attachments.Add(attachment)
Edit-
You are assigning a Text value to the Boolean property. So convert the text to Boolen first
e_mail.IsBodyHtml = cbohtmlbody.Text 'cannot be read
should be
e_mail.IsBodyHtml = Convert.ToBoolean(cbohtmlbody.Text) 'cannot be read
Same for other errors too..
Similarly
Smtp_Server.Host = cboserhost.Text 'cannot be read
Should be
Smtp_Server.Host = cboserhost.SelectedItem.Text 'cannot be read
And do not jump into programming without learning the basics