Networks can be easier to secure - if there is a single computer (or a small group of computers) which hold the sensitive information, those are the computers which must be physically secured. Other computers can be left less secure, provided the network itself is secure and the unsecured computers don't have sensitive information on them - such as network passwords.
In 'big business' the computers which store the sensitive information are often kept in a special computer room, in a secured building. In small business or home environments, keep these separate - don't use them as regular computers. Make certain they're behind the scenes somewhere, away from customers.