It is very important to be careful about downloading antivirus packages off of the Internet. Before downloading a package, you should do some research to find out if it really does what it claims to do. Some antivirus packages that advertise via popups will let you run an on-line antivirus to check out your system and then falsely report to you that you have viruses that are not present and may easily miss viruses that you have. If you get a popup advertisement telling you that you have a virus, do not use the software being advertised to scan for the virus.
It is inadvisable to try to use more than one antivirus package at a time on the same computer. When installing new antivirus software, make sure that you first remove any previous antivirus software you are already running.