This actually happened to me a couple of times with the main Creatures 1 executable. I got quite worried and ended up doing a bunch of scans across my entire system. Definitely a false positive! I think the main issue is that the file name C1 runs from is called "Launcher.exe" which sounds pretty vague. Likely Avast associates it with viruses and the like. It's not a bad idea to always run a quick scan whenever something like this happens, but I can vouch for the fact that Creatures 1 will sometimes be flagged. I haven't tried changing the file name, although adding it as an exception in Avast seems to do the trick!
Thanks for posting about this! I'm sure others have experienced it. Nothing quite like a little panic when you just want to play Creatures. Ha!