title: PyCharm asks for python interpreter every time project is loaded
body: <p>Rather frustratingly, every time I load up an existing project or create a new one - it complains there is no interpreter selected and I have to provide the path to it.</p>
  
  <p>There is nothing unusual about my python installation, it's sat in C:/Python27/ as you would expect. It always used to work, but the last few weeks something has changed.</p>
  
  <p>I'm running Win7. PATH system variable points to C:/Python27/ as it ought to.</p>
  
  <p>If I crack open commandline, the python command opens as it ought to.</p>
  
  <p>Any ideas?</p>