Can I turn off my cable/DSL modem when not in use ?

Most service providers suggest that modems remain on at all times. The provider might have to upgrade firmware, monitor the network for problems, etc. If security is the concern, you can simply unplug the network cable connected to your PC instead, use firewall, etc. Even though there are no serious technical problems with turning the modem off, it is probably better to follow the service providers' suggestion.