I guess you got banned from some message board, huh?
If your ISP is set up properly, you won't be able to change your IP. If you have cable/DSL and are directly connected to the modem (no router in between), you can try entering the following at the command line:
ipconfig /release
ipconfig /renew
It probably won't work, though. ISP map your MAC address to IPs so that you can't start gobbling up random addresses, and you can't manually change your IP address and still expect your connection to work properly.
And I'm not telling you the other ways to do it because homos here will read that and try it out.