It is important to know whether it is a modem or a router as there is a distinct difference in behaviour between the two. I'm guessing, as you point out that your PC is connected to it too, that it is a router rather than a modem.
by Alan
I connected her PC to the modem/router with the supplied cables, still leaving mine connected.
If so, I wouldn't have expected you to need to install any software at all. Most home routers are automatically configured to use DHCP. With all participating machines connected to the router via network cables (most routers allow about 4 or so machines to connect) and the TCP/IP network settings on each PC configured to "Obtain an IP Address automatically" that the machines should simply be able to connect without installing or configuring anything else. The router has the job of maintaining an open connection with your broadband provider and it is then able to "route" individual connection requests from each PC and make sure that the right answers are passed back to the correct machine.
As far as the problem with the missing files is concerned, I'm afraid to say that the best option, as far as I know, is to re-install Windows (the classic Microsoft solution to all ills). It will, unfortunately, mean that most of the applications will need to be re-installed too, but as long as you don't re-format the hard disk all the data files should remain.