On a computer that is running Windows Vista, Windows Media Center displays DVD content that is stored on your computer in the DVD Library. However, the DVD Library is not automatically enabled. If it is not enabled, you cannot see your DVD content.

To enable the DVD Library, follow these steps:

1. If Windows Media Center is running, close it.
2. Click Start
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   .
3. In the Start Search box, paste or type regedit.
4. In the Programs list, click regedit.
   
   
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   If you are prompted for an administrator password or confirmation, type your password, or click Continue.
5. Locate and then click the following registry subkey:
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Media Center\Settings\DvdSettings
6. In the details pane, right-click ShowGallery, and then click Modify.
7. Delete the contents that appear in the Value data box.
8. In the Value data box, type Gallery, and then click OK.
9. Close Registry Editor.

or

To fix this problem automatically, click the Fix this problem link. Then, click Run in the File Download dialog box and follow the steps in this wizard.

Fix this problem

Source- Microsoft Help and support

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