This is probably a bug with a thread in Bug Reports a while back that Juan thinks he has solved and will include in the next update. If you open PI, do not do anything else, and shut it down either with the exit option or clicking in the right corner, PI should reopen ok. If you open PI and launch Process Explorer, close it and exit PI by clicking on the upper right box, you will not be able to relaunch PI till you reboot because there is a Pixinsight process still running after PI is closed. If you close that running process in Task Manager, you will be able to launch PI again...