Whether it's a cable or DSL modem installation, they'll just be doing the wiring, hooking up the modem and that's it. Rarely do they ever do anything with home networking - fuck that. It's your responsibility to setup your cheap wireless router. The furthest they'll go in terms of testing your connectivity is making sure your web browser connects to the default home page and that's that.
You have nothing to worry about. They'll sooner steal a small valuable possession than snoop around on your computer. Installers are usually unit based and make commissions for all the different products they install. It's better for them to just get things going nice and quickly and move onto the next customer.