Mathcad 11 does not start

I have just installed Mathcad 11 on my freshly reinstalled Windows XP and it does not function !!! It appears in the task manager dialog as a running process, but that is all. Has anyone an idea why ?
Same think happened, when I installed Mathcad 12 before. S..t !!!!!

Please, if you know, what it could be, let me know.

David