Do I need USB isolator for laptop computer?
I suggest you consider isolating the RS-232 as @Ignacio suggests. You can even buy a packaged single-chip solution that actually includes internal transformers for signal and power isolation: 



A USB port in your unit with a connection to the mains is an extremely dangerous thing to manufacture- someday someone may plug something into that plug that will kill them or destroy a desktop computer. 

With a laptop computer, depending on how it connects to the mains, touching the shell on another USB plug or headphone jack etc. when removing it or plugging it in could kill someone. This has happened for real, it is not just a theoretical risk.