How do i keep my son out of the windows?

I have an 18-month-old son. He's not even close to being your average child. We have windows upstairs, and they have ledges big enough for him to stand in and walk along. every chance he gets he's walking on them. at first i thought he was climbing up onto them from our bed, so i moved it. he is still able to get in them though. he just grabs a hold of the edge and swings himself up. I have looked at a lot of babyproofing stuff, but haven't been able to find anything to keep him out of the windows. Has anyone else had a similar problem? What worked for you? I'm really afraid he's going to fall out when i'm showering or something.