First step: Breathe. Hang in there.
If there was a problem, the program already "has been prevented from running" by whichever Windows service stopped it.
I suspect that there's no problem at all -- which is good news for your personal info! I know from experience that Windows Defender tends to block most independent software (and each new version of that software, every time) when you first try to run them. It's usually not difficult to tell it to let the program run anyway if you know what it is and if you trust the source of the files.
(I don't mind having to tell Windows that my usual open-source programs are okay if it keeps weird mystery programs from running on my Grandma's laptop, y'know?)
In this case the phrase "or does something potentially unwanted" may just be Decker's ability to create deck files. Creating new files would be worrying if it was actually some kind of malware, but in the case of creative software it's quite necessary to be able to save your work.
For what it's worth, I had Windows Defender scan my copies of Decker 1.52 as a program and in it's zip that I downloaded here on itch.io and it didn't find anything suspicious or unusual. And despite finding nothing weird it still prevented the new version of Decker from running the first time I tried to open it. Because it always does. And I told it to "Run anyway", as always. And everything about Decker 1.52 seems completely fine and normal here.
It's good to be safe! It's a very good practice to change your email passwords occasionally, whether or not there's a problem. But I think there probably isn't one this time.
If you're still concerned, you can always build Decker from source. Here's the link to Decker's GitHub.