My daughter went through a "fun" phase where she was curious about (and afraid of) all the things that could get in the house. We used Febreze to spray for everything from bugs to witches to monsters. To avoid having to spray every night, we told her it was a long acting spray and only needed to be reapplied every month or so. By the time we were supposed to "reapply" she was over that particular fear. By then she had moved on to bears, pirates, alligators...
I disagree with the people that say don't acknowledge the fear and tell them this stuff is not real. In a child's mind these things do exist and by dismissing their fears it does not help them to deal with them. We acknowledged every imagining of my daughter by telling her that we understand that she is afraid of these things, explained to her how they could not hurt her (alligators cannot turn doorknobs after all, and pirates travel by boat and we don't live near water, etc) to help her find ways to dispel them herself and problem solve. This worked way better than any other method and after a while she would come to us and say that she was afraid of something (bees, for example) but realized they could not get her because they can't open windows.
We also got the book "There Is A Monster At The End Of This Book" where Grover is scared to let you keep reading the book because there is a monster at the end. I don't want to spoil it for you, but as you get to the end, the only one there is Grover and he realizes that he "sweet lovable furry Grover" is the monster at the end of the book.
Good luck... have fun with this phase. We did, and now we have a happy, imaginitive and well adjusted 5 year old who is capable of dispelling her own fears through imagination and play.
M.