Several add-ons throw error "DLL load failed. %1 is not a valid Win32 application." Known add-ons with this issue are Tissue, RetopoFlow, and XNALara/XPS Import/Export.  I am running Blender 2.79 on Windows 10 64-bit.  I have also experienced this issue in 2.78, but not in 2.77.  

Edit: This also occurs when I try to switch themes.
Try the following:


Close Blender
Rename C:\Users\YourNameHere\AppData\Roaming\Blender
Foundation\Blender\2.79 to
C:\Users\YourNameHere\AppData\Roaming\Blender
Foundation\Blender\2.79.bak
Use your login in Windows instead of YourNameHere and your version
of Blender instead of 2.79
Start Blender again