If your connection is direct to your computer and your computer gets the public IP and not a router, you can try this:
For Windows 2000, XP, and 2003
1. Click Start
2. Click Run
3. Type in cmd and hit ok
4. Type ipconfig /release and hit enter
5. Click Start, Control Panel, and open Network Connections
6. Find and Right click on the active Local Area Connection and choose Properties
7. Double-click on the Internet Protocol (TCP/IP)
8. Click on Use the following IP address
9. Enter a false IP like 123.123.123.123
10. Press Tab and the Subnet Mask section will populate with default numbers
11. Hit OK twice
12. Right click the active Local Area Connection again and choose Properties
13. Double-click on the Internet Protocol (TCP/IP)
14. Choose Obtain an IP address automatically
15. Hit OK twice
16. Go to What Is My IP to see if you have a new IP address

For Vista
1. Click Start
2. Click All Programs expand the Accessories menu
3. In the Accessories menu, Right Click Command Prompt and choose Run as administrator
4. Type ipconfig /release and hit enter
5. Click Start, Control Panel, and open Network and Sharing Center. Depending on your view, you may have to click Network and Internet before you see the Network and Sharing Center icon
6. From the Tasks menu on the left, choose Manage Network Connections
7. Find and Right click on the active Local Area Connection and choose Properties (If you’re hit with a UAC prompt, choose Continue)
8. Double-click on Internet Protocol Version 4 (TCP/IPv4)
9. Click on Use the following IP address
10. Enter a false IP like 123.123.123.123
11. Press Tab and the Subnet Mask section will populate with default numbers
12. Hit OK twice
13. Right click the active Local Area Connection again and choose Properties
14. Double-click on Internet Protocol Version 4 (TCP/IPv4)
15. Choose Obtain an IP address automatically
16. Hit OK twice
17. Go to What Is My IP to see if you have a new IP address