I have this exact same problem.
Over time it also gets worse...

After some time SED will stop opening on my computer.
It's process only shows up in the list of running processes.
The only way to get SED to open for me after this period of time is to reformat and install gamestudio.

So when this happens I usually switch to MSVC++ or GSTBuilder.
Although GSTBuilder erases your code if you open a file but don't make changes to it or save it then close it.