This is why I use
BlueIDE, and probably will until it's no longer compatible. I don't know the development history for Synergy, but it's never really been my cup of tea.
I'm still watching the new
Indigo editor, as it's being developed by the same person that made BlueIDE. I may switch to that once it's a little further along.
To answer your question though, EditorNew.exe brings up the latest Synergy editor for me. (running Windows 7 64bit) You might try the typical settings for many applications since Vista. Set to run as Adminisrator, and run it under XP SP3 Compatibility with "Disable visual themes" and "Disable desktop composition" checked.