One of those stupid gotchas which you struggle with for ages and the answer was staring you in the face all along. I recently got a new keyboard and installed the accompanying Intellipoint software. The first time I tried to run a SQL query with F5, it tried to open a file instead! I wasted a lot of time Googling for an answer, but I should have just looked at the keyboard more carefully – there is an “F lock” key at the right-hand side of the F-keys row. Sure enough, once I’d turned F-lock on, my F5 starting executing queries again. What a relief.