If you're cable modem, some of the Motorola modems have a 'standby' button on the top that completely disables internet access until the button is pushed again. Doesn't stop network access, however. I use zone-alarm myself, and have the 'internet lock' set to shut down ALL access to the PC after 20 minutes of inactivity.
You could also try an A/B switch from an electronics store. Put the PC on 'A' and leave 'B' empty. Switch to 'B' when you want to be secure.
-R