The only antivirus I ever bought was Norton's, and in addition to being a general pain in the butt, I found out it was constantly spinning the hard drives. Which is bad, very bad. So my choice was Nortons or nothing, and I chose nothing!

There's really nothing malicious about it. It writes to disk (when you save) and reads files from disk (when you add sprites). It connects to the Internet to check for updates and so on. Nothing too crazy. Make sure you're downloading it from here: http://starboundsoftware.com/software/realm-factory/. (You probably are.)

The only thing I can think of is that when you download files from the Internet, Windows knows, and is very nervous about running them. (I suspect Symantec is doing a very similar thing.)

The way to fix this is probably to "unblock" the file. This can be done by right-clicking on the file and looking on the Security tab (or something like that… I'm going from memory here…) towards the bottom for an Unblock button.

Actually, come to think of it, I think I've already written about that somewhere around here…

Yep, there it is! http://rbwhitaker.wikidot.com/forum/t-464766/getting-an-error#post-1431820

If that doesn't do it for you, I don't know what to tell you. You could maybe figure out how to get Symantec to make an exception for this file or turn it off temporarily (kind of a bad idea) or something.