I recently reinstalled my Windows 7 Home Premium and installed the latest Updates.
Yesterday I installed both the latest version (3.0.1) and the 2.9 version of Krita (both the x64 because my Windows is 64 bit).
But both installed and portable won't start at all when I run the executable. Nothing shows up.

My specs are:
OS: (as mentioned) Windows 7 Home Premium 64 bit, Service Pack 1 (runs on a SSD)
CPU: Intel Core i5-3380M @2.90GHz
RAM: 16,0 GB
GPU: GTX 660M 2GB

If anyone has a clue or a solution I'd be really thankful.

Have you tried rebooting?

It might be helpful if you can post some screenshots on how you are trying to start Krita.

I tried rebooting several times.

Here are the screenshots:
https://drive.google.com/open?id=0B-oQ25CD1N4oSUU4SUd2M2hEZ2M
https://drive.google.com/open?id=0B-oQ25CD1N4oWENBZFZDb2FaZUE

If you start Task Manager and look at the "process" tab, do you see any krita.exe? Or does it just show up for a split second when you tried to start Krita?

It just shows up for a second.