OneNote 2010 opens a folder as a notebook if there is no .onetoc2 file located in the folder

To resolve this issue, follow these steps:  

1. Apply the hotfix package that is described in Microsoft Knowledge Base (KB) article 2523130. For more information, click the following article number to view the article in the Microsoft Knowledge Base:  
   2523130 Description of the OneNote 2010 hotfix package (onenote-x-none.msp): April 26, 2011
2. Enable the hotfix package. To do this, follow these steps:
   1. In Windows 7 or in Windows Vista, click Start, type regedit in the Start Search box, and then press ENTER.
      
      
      If you are prompted for an administrator password, type the password. If you are prompted for confirmation, provide confirmation.
      
      
      In Windows XP, click Start, click Run, type regedit in the Open box, and then click OK.
   2. Locate and then click the following registry subkey:
      
      
      HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\OneNote\Options\
   3. On the Edit menu, point to New, and then click DWORD Value.
   4. Type EnableOpenFolderAsNotebook, and then press ENTER.
   5. Right-click EnableOpenFolderAsNotebook, and then click Modify.
   6. In the Value data box, type 0, and then click OK.
   7. Exit Registry Editor.

Now, if no .onetoc2 file is found in the folder, OneNote 2010 opens a single section. If a .onetoc2 file is found in the folder, OneNote 2010 opens the .onetoc2 file as a notebook.

Note Additionally, this update improves performance and synchronization against SharePoint servers.