officially, it's unsupported, and you should consider updating to get support for modern OS versions (or downgrade back to windows 7/8 which are supported).
The other obvious things to try would be:
1) Install the latest update to filemaker after reinstallation (13.0v9)
2) Switch from the 32 to the 64 bit version (or vice versa)
3) Run filemaker 13 in a virtual machine running a compatible older OS
Have you tried right clicking on an empty spot on the task bar? Then click "Cascade Windows" that should get the windows that are off-screen, back onto the viewable area.
Thanks for your responses. Joshua, your instructions worked like a dream.