I have Mcafee on trial at the moment,wich is due to finish shortly, can I then safely download another antivirus product say avg without uninstalling mcafee ? be very grateful for any help.
You can safely download any file, including the the set-up file for AVG (or Avast! or whatever).
However, you should never have two anti-virus programs (or two firewalls) running simultaneously. You should uninstall McAfee (using the tool I linked to when you previously posted about McAfee) before installing your new anti-virus software.