Are you talking about Moto Helper? Motocast requires a manual download of program from Motorola so unless you already downloaded it, shouldn't be any prompts for install I've ever seen.
My Razr Maxx does the samething. Whenever i connect it to my PC via a USB port, the MotoCast auto-installer starts downloading (on the PC, not on the phone). It there a way to stop this without installing Motocast?