Hi,
Yes, the "turning modem off overnight" thing rarely works anymore. ISP's that allow that end of with abusive users frequently changing IP's.
If you need to change your IP address frequently, use a fee based service like
http://www.iprivacytools.com to simplify your life.
If you want to just get your IP changed once, try this:
1) Click your START menu, type "cmd" and Enter.
2) In box, type "ipconfig /release" and Enter.
3) Right-click your "local area connection" and select "properties".
4) Doubleclick "Internet Protocol tcp/ip".
5) Check "Use the following IP address" and enter 111.111.111.111 for ip and 255.0.0.0 for subnet mask.
6) Click "OK" and close the panel.
7) Wait a few minutes, then go back into that dialog box and check "obtain an IP address automatically".
8) Surf to myipaddress.com to see if your ip changed
If that doesn't work, contact your ISP directly. Tell them you're having trouble with your IP address and if they can tell you the process for being issued a new one.