If you have nVIDIA graphics adapter and installed nView Desktop Manager and you also have VirtualBox running in the host machine then Alt+Tab functionality in the guest will affected by nView if it is enabled.
This was observed on:
OS: Windows 8.1 Enterprise 64-bit
PyCharm can sometimes get confused specially if you have multiple interpreters and using virtualenv. To fix "Unresolved reference 'print'" message, do File | Setting... (Ctrl+Alt+S) | Project Interpreter | Show All (drop down) | Show path for the selected interpreter () | Reload List of Paths ().