Installing SQL Server Express in parallel of Standard
You are on  a totally wrong path here. Seriously.


  So i suspect either SQL SErver is corrupted or windows is corrupted (other ideas?).


Why? To be THAT corrupt (2 seconds to 45 minutes) would basically mean not running. Event log full of errors?

Check performance counters. I would say totally overloaded disc subsystem or too little memory - a real normal debug situation, nothing else. Installing Express won't solve anything.

To answer your question.

Yes, it is possible. It is another instance with a predefined name (SQLExpress).

But you are much better on to sit down and do beginner baseline analysis - performance counters etc. SQL Server pretty much can not be THAT currupt as you indicate and still work. I think that is a purely performance issue and should be dealt with it - logically, not with blind actionism.