Found at http://support.microsoft.com/kb/899711

The page cannot be found HTTP 404 - File not found.

In SharePoint Portal Server 2003 portal site or a Windows SharePoint Services Web site

CAUSE
This issue occurs if you do not configure an exclusion for the path of the new virtual directory. If you do not want SharePoint Portal Server 2003 or Windows SharePoint Services to handle requests for content in the virtual directory, you must exclude the path of the virtual directory.

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RESOLUTION
To resolve this issue, configure an exclusion for the path of the new virtual directory. Follow these steps:
1. Start Windows SharePoint Services Central Administration.
2. Under Virtual Server Configuration on the Windows SharePoint Services Central Administration page, click Configure virtual server settings.
3. On the Virtual Server List page, click the virtual server that you want to configure. For example, click Default Web Site.
4. Under Virtual Server Management on the Virtual Server Settings page, click Define managed paths.
5. In the Add a New Path area of the Define Managed Paths page, type the path that you want to exclude from Windows SharePoint Services in the Path box. For example, type /NameOfVirtualDirectory.
6. Under Type, click Excluded path, and then click OK.
7. Verify that the path that you excluded is listed in the Excluded paths area of the Define Managed Paths page.