Hello, Ljoel. Welcome to Bleeping Computer.
We must first clean up your .NET framework installation. Download and run this file
. It will remove the .NET framework from your computer so that we can reinstall it later (and solve the non-installing issue). My sister's laptop did the same, as well as our home desktop (failed install). But don't worry, it is very easy to fix.
After running this cleanup tool, it will ask you to reboot. Please do so.
Next, download and install this file
. This will reinstall the .NET framework onto your computer.
After another reboot, download and install this file
. This will update the .NET framework to what it should be.
Does this solve your problem?