Error Message: Error 633 Modem Is Already in Use


When you try to use your modem on a Windows XP-based computer, you may receive an error message similar to the following:
Error 633 Modem is already in use.


This behavior may occur when another device with a driver that is incompatible with Windows XP is in conflict with the modem. For example, this problem can occur with outdated Palm Pilot software.


To resolve this issue, follow these steps:

  1. In safe mode, remove all the modem drivers, and then restart the computer.

    Windows will redetect the modem.
  2. Click Start, and then click Control Panel.
  3. Click Add or Remove Programs.
  4. Click the incompatible software, for example, Palm Desktop and Synchronization Software.
  5. Click Change/Remove, and then follow the steps to remove the software.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

More Information

For additional information about Palm Pilot software and Windows XP, click the article number below to view the article in the Microsoft Knowledge Base:

308368 Windows XP Upgrade Advisor Warning About the IrDA Protocol
For additional information about troubleshooting modem problems, click the article number below to view the article in the Microsoft Knowledge Base:

308022 Resources for Troubleshooting Modem Problems in Windows XP
The third-party products that are discussed in this article are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, regarding the performance or reliability of these products.

Microsoft provides third-party contact information to help you find technical support. This contact information may change without notice. Microsoft does not guarantee the accuracy of this third-party contact information.

Article ID: 324760 - Last Review: Jul 31, 2014 - Revision: 1

Microsoft Windows XP Professional, Microsoft Windows XP Home Edition