Why do I need to restart my modem when I switch the cable between my PC and Mac?

each device pulls a seperate IP address. Once you modem picks up one many cannot automatically switch between devices. You can get a cheap router($40) that will split that signal so you do not have to reset. A router works like a cable splitter for your TV.....

Wireless will help as long as both your PC and Mac have a wirelss card. About $60 a piece

Because the modem creates a table for your access to the network which stores that MAC address of your Ethernet card. Every MAC address is unique to its Ethernet card. There are no two in the world that are the same. So when you switch from one computer to another, the modem is looking for the wrong Ethernet card, until you reset it and it finds the MAC address for the other card. This is true for any Mac computer or PC. If you were to switch between two separate PCs you would still have to reset the modem.

Probably because the IP address given out by your internet provider is DHCP which means that it is handed out by a server and every time you switch the cable it has to pick up the new MAC address on each machine and must renew the IP lease to the machine. The way it renews the lease is a restart...to fix this you might want to get a router and and extra cable...that way you won't have to be restarting every time and you won't have to be switching cables out.

Are both of your computers wireless ready? Going wireless will solve allot of problems, and you can have them both online, even though you use them at different times. You can also get a router, and hook both the pc and mac up via eathernet. A router that will work well with mac and pc is linksys, I have used my mac and pc on it before.