I use Inquisit on Win7. Sometimes during the experiment Windows taskbar pops up and remains on the bottom of the screen while the visual stimuli are presented. How to avoid that?

This would only happen if some other application running in the background activates the taskbar (because it starts running, or displays some notification, etc.). There really isn't anything Inquisit can do to prevent that. You'll have to find out what exactly causes the taskbar to activate and then mitigate that. See e.g. http://www.sevenforums.com/software/213108-taskbar-appears-while-watching-fullscreen-video.html or https://social.technet.microsoft.com/Forums/office/en-US/23ac26bc-4732-4f19-ba24-9a54fd712a2f/my-taskbar-keeps-popping-up-in-windows-7-whats-causing-it-and-how-do-i-stop-it?forum=itprovistadesktopui for some things you can check.