well what's happened is that the server at the cable company took the ip address off your desktop and basically said, that's the only computer I'll recognize.
To resolve this, you can do a couple of things. First is get a wired or wireless router, whichever one you want to use. Turn off the cable modem [power it off]. hook up the cable modem ethernet cable to the WAN port of the wired or wireless router.
Then turn on your cable modem and turn on your wired or wireless router. Then connect your computers to the wired or wireless router.
you might have to enter your wired or wireless router and in the setup section, click the release button, then click the renew button.
What happens is that if you connect your PC directly to the cable modem, the cable ISP sees your computer and records your computers MAC address. Then only your computer can connect. turning off your cable modem, using a consumer level router, sending a release command to the cable providers servers and then sending a renew command gets rid of your PCs MAC address.
