Go to the start menu and open up the "my computer" tab. From here, on the left hand side, you will see a few mini links and one of them should say "Network Places" or something along those lines. Open that link and you will see an icon that is titled "LAN". Right click that and then select the "Properties" option. A small window will pop up, you're going to want to scroll down in the box until you see the "TCP/IP connections" and then highlight that and select the Properties radio button. From here you will see a window that allows you to change your IP and DNS settings.
And you don't have to wait a few hours you can turn it back on instantly. For example with my DSL I can turn my modem on/off and instantly get a new IP.
However if the admin of whatever ban I'm trying to get around is smart they will still find me and be able to ban me because my IP will be within a certain subnet. What does that mean?
Example my ip would come in this range:
1.1.1.1
1.1.1.2
1.1.1.3
...etc
1.1.1.120
So they would just ban the whole subnet that I could be in. This could still result in them banning some innocent people that live near me but the odds of that mattering are extremely low if you consider the amount of people around you and the odds of them visiting that site etc.
The best way to change your IP is to use a proxy. /end thread.