I've heard a lot about securing a network from the inside-out, assuming that people will have access no matter what. Obviously some simple measures can be put in place to deter casual passers-by, but if someone really wants in, can they be stopped?
Assuming no, what can be done (with hardware or software) to protect the system from abuse once someone has access to the router? Can I protect my Windows machine, for example, from someone while still being able to access the internet and my shared files?