There's not really a perfect way to do this. You can use sp_init() in Global Lua, but that will fire every time S+ is reinitialized, which happens when you click Apply/OK in the S+ window or select Reload from the tray icon.
Also, S+ doesn't know when Windows is "ready to be used". There's probably some way to do this, using Scheduled Tasks or something, but it's not a simple thing built into S+.