The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Thats nothing to worry about. When windows shows you the warning screen, there's an option to tell you more or run it anyway - something like that it's been a while since i've seen it. Basically you can override this warning and then install it. There is no virus. Windows is trying to be helpful but just being annoying.
