The IP address that you're assigned, is bound to the MAC address of your network card. There's several ways to change your IP, without contacting your ISP.
1) Disconnect your computer from the modem, wait for your DHCP lease to expire, and then reconnect the computer. This is the least convenient method, since it usually requires waiting several days.
2) Connect a router to your modem (or if you're already using a router, disconnect it)
3) Use a different network card to connect to the modem, or use the USB cable instead of ethernet (driver install required for USB)
4) Change the MAC Address of your network card. You don't need any special software to do this. In Windows XP, you can do this through Device Manager ( usually an advanced setting such as "Network Address"), or in Win 98 you can do it through the Network control panel
After any of these changes, you will most likely need to reset your cable modem, by unplugging the power for 30 seconds, and plugging it back in.