Issue pyinstaller#521: On Windows extend system PATH for PyQt4 hook.

PyInstaller/compat.py

@@ -339,7 +339,8 @@ def __add_obsolete_options(parser):
 # Site-packages functions - use native function if available.
 if hasattr(site, 'getsitepackages'):
     getsitepackages = site.getsitepackages
-# Backported For Python 2.6.
+# Backported For Python 2.6 and virtualenv.
+# Module 'site' in virtualenv might not have this attribute.
 else:
     def getsitepackages():
         """
@@ -350,7 +351,8 @@ def getsitepackages():
             pths = [os.path.join(sys.prefix, 'Lib', 'site-packages')]
             # Include Real sys.prefix for virtualenv.
             if is_virtualenv:
-                pths.append[os.path.join(venv_real_prefix, 'Lib', 'site-packages')]
+                pths.append(os.path.join(venv_real_prefix, 'Lib', 'site-packages'))
+            return pths
         else:
             # TODO Implement for Python 2.6 on other platforms.
             raise NotImplementedError()

PyInstaller/hooks/hook-PyQt4.py

@@ -8,8 +8,18 @@
 #-----------------------------------------------------------------------------
 
 
+import os
+
 from PyInstaller.hooks.hookutils import qt4_menu_nib_dir
-from PyInstaller.compat import is_darwin
+from PyInstaller.compat import getsitepackages, is_darwin, is_win
+
+
+# On Windows system PATH has to be extended to point to the PyQt4 directory.
+# The PySide directory contains Qt dlls. We need to avoid including different
+# version of Qt libraries when there is installed another application (e.g. QtCreator)
+if is_win:
+    from PyInstaller.utils.winutils import extend_system_path
+    extend_system_path([os.path.join(x, 'PyQt4') for x in getsitepackages()])
 
 
 # In the new consolidated mode any PyQt depends on _qt