Importing the REG file did not change anything.
I have dowloaded and installed the x86 version. The Program Compatiblity Assistant Service was on automatic. I changed to Manual, as suggested. It was already running.
In the Properties of MathAppx86 I have checked and Compatibility Mode box and chosen Windows 7
No improvement. I see the splashscreen for a while. And then it quits. This time a Troubleshoort fires up, suggesting me to "Test the program..." I chose that option but nothing happened. After clickng on "Next" it saved some settings, saying "Incompatible Program Fixed". No change, same behaviour.