If you are experiencing this when trying to use Dialup Networking (ThinkPad Modem) or Verizon VZAccess Broadband, the following may be useful. Note, the solution involves modifying the registry on XP so it is not for the faint of heart.
Note that a key symptom of this problem is that the modem connects and then disconnects just after trying to register the computer on the remote network.
Step 1: Uninstall the WAN Miniport (IP) and WAN Miniport (PPTP)
IMPORTANT: This procedure contains information about modifying the registry.
Before you modify the registry, make sure to back it up and make sure that
you understand how to restore the registry if a problem occurs. For
information about how to back up, restore, and edit the registry, click the following
article number to view the article in the Microsoft Knowledge Base: 256986
(http://support.microsoft.com/kb/256986/EN-US/) Description of the Microsoft Windows Registry
When you try to use a dial-up connection to connect to your Internet Service
Provider (ISP), you may receive the following error message during the
"Registering your computer on the network" portion of the connection
Error 720 - the connection attempt failed because your computer and the
remote computer could not agree on PPP control protocol.
Additionally, when you view the devices in Device Manager, you may notice a
yellow exclamation mark (!) over the icon for the WAN Miniport IP (#2) device.
This issue may occur if the TCP/IP protocol has become unbound from the
dial-up adapter, or if a conflict is created when a second WAN Miniport IP
device driver loads.
To resolve this issue, uninstall the WAN Miniport IP and PPTP devices.
To do so, follow these steps:
1. Right-click My Computer, and then click Properties.
2. Click the Hardware tab, and then click Device Manager.
3. On the View menu, click Show hidden devices.
4. Under Network adapters, you will see WAN Miniport IP
devices. If a WAN Miniport IP device is listed, continue to the
following step. If no WAN Miniport IP device is listed, go directly
to step 16.
5. Click Start, click Run, type regedit in the Open box, and then click OK.
WARNING: If you use Registry Editor incorrectly, you may cause serious
problems that may require you to reinstall your operating system. Microsoft
cannot guarantee that you can solve problems that result from using Registry
Editor incorrectly. Use Registry Editor at your own risk.