Do you mean the IP given out by your ISP? From my understanding this IP is handled by the ISP and there is no way to change it (unless its a DHCP lease and they let you renew the lease and get a new IP). Even most ISPs that use DHCP will force you to reuse the same lease until it expires.
cable IPs aren't always static. At Comcast, they are dynamic, however the lease for long periods of time or sometimes they renew with the same IP. If you want to change the IP that cable gives you, the best way is to unplug the modem for a period of 3 hours or better. When I want to change mine, I just unplug before I go to sleep.
there some ways working under XP and Vista, but the one under XP does not work under Vista, which will not most probably work under Win 7, and the one working under Vista requires to unplug the modem. i'm looking for a way that does not require to unplug the modem. thanx for the answers anyway.