I don't know why or how you are seeing that. I tried the site on three different machines.

The files are downloaded as ZIP files, so there is no way your anti-virus program would view them as containing viruses. The download would have to complete first for the antivirus program to even scan the files.

It sounds like you may have a plug-in or something on your computer that causes this message to appear or a local virus.

You should be downloading from : http://www.realflight.com/downloads/add-ons.html