Remove the security software and install MSE, microsoft security essential, which is their own software.
This is exactly what anti-virus is supposed to warn about, nothing wrong there. Anything you download from the internet 'could' be a threat. If it updated from tpt itself, or got it from the main download page, just hit allow, it is doing its job. I use Avast! and it does this as well every update.
Not a good idea. A virus could somehow sneak past and you won't realise it until it's too late. (But the chances are low)
And then wait for your passwords to be stolen and your computer to be destroyed!!
Honestly, I use Symantec Endpoint Protection anti-virus and I have no problem with TPT being mentioned as a virus when I download.
How can TPT be a virus?
IT can be, if you downloaded it from a site you don't know, that's unofficial, the chances are high but if you download it from this website, it's 100% safe.
Also, can't you just download the latest version from here?
https://powdertoy.co.uk/Download.html
Besides, there's no beta (as of this post)