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For cable and DSL on any recent version of Windows:

1. Make sure your computer is hooked up directly to your modem. (No router)
2. Go to Start -> Run cmd.exe
3. Type ipconfig /release
4. Press the reset button on your modem and wait for it to reboot. (Button usually is in a small hole in the back)
5. Type ipconfig /renew

Chances are good you'll get a fresh IP.

If you have a modem/router, just resetting it should do the trick. Unless you have a static IP, then you're in a bit of trouble (you'll need to go down the proxy route)

Bump for new info.

with a router, or a "box", you need to turn off your internet, then turn it on again. Restarting the box is bossible too, but longer in time.

Try to empty your cookiez, it will be useful.