If you are comparing something like Virtualbox or VMware to this, it isn't the same thing. This is a security sandbox, meant to not allow any changes to your system from things within it. You can maybe Run something in it, but installation is part of what it is made to prevent.
As for if FPScreator is safe, I have heard of it before, its not unknown to me. Most likely cause for Avast! wanting to autosandbox it is the file not being digitally signed. But if you downloaded it from a legit source, it should be safe.
