Hi Ari94,
I am Dave, I will help you with this.
.Net Framework 3.5 contains the code base of .net 2.0 and 1.1
Open the old Control Panel, then at the top set View to icons
Open Programs and Features, then click 'Turn Windows Features on or off'
Form there, you can install .Net Framework 3.5
Then restart your PC to apply the change.
If that does not get that software working, the developer needs to upgrade their code.