Welcome to the Doom9 forums, tarrega
The error you have is very strange. It seems that it is caused by a missing file, probably located in the Windows directory. Where is Windows located in your system ? Usually, it's in C:\windows, and BD3D2MK3D uses the environment variable %WINDIR% to discover it. Maybe there is something wrong here.
Please open a command prompt window and type this:
echo "%WINDIR%"
On most systems, it replies this:
"C:\Windows"
Let me know what it has replied for you.
Also, the only thing that BD3D2MK3D searches now in the Windows directory is if .NET v4 or greater is installed (as it's necessary to hardcode the subtitles on the video). So, please open this directory in the File Explorer:
C:\Windows\Microsoft.NET\Framework\
And let me know if there is at least one folder beginning with "v4." And if it's not the case, please install
Microsoft .NET V4 (currently, v4.8 is the latest version).
Also, in the big white window where the error message is shown, after the prompt () 1 %, type this:
IsNetV4Installed
Normally, you should see "true" if .NET v4 is already installed, or "false" otherwise. But if that command produces the error message you have noticed, that will confirm my suspicion that it's the .NET checking function that is the culprit, and I should be able to fix the bug.
Also, be sure to use the latest version of BD3D2MK3D (v1.17). I have recently fixed another bug that can also explain the problem.