Rex - thanks for the good advice.  I haven't seen this problem myself, but I'd assume if WLPG or Windows crashed during a write the the database, it could become corrupt.  Here is the solution from the link Rex posted.  Please note the WLPG 2009's database is called Pictures.pd5

 To resolve this problem, you must force Windows Live Photo Gallery to regenerate the database. To do this, follow the steps for your operating system:

• Windows Vista
  1. Close Windows Live Photo Gallery.
  2. Click Start
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     , type %userprofile%\AppData\Local\Microsoft\Windows Live Photo Gallery in the Start Search box, and then press ENTER.
  3. Right-click Pictures.pd4, and then select Rename.
  4. Change the name of file to OLD_Pictures.pd4.
  5. Start Windows Live Photo Gallery.
  Note Windows Live Photo Gallery will re-create the database based on the metadata and information that is stored in your media files. Any metadata that is not written back to a media file will be lost. For example, the metadata could be lost because a media file was locked or read-only.
• Windows XP
  1. Close Windows Live Photo Gallery
  2. Click Start, click Run, type %userprofile%\Local Settings\Application Data\Microsoft\Windows Live Photo Gallery, and then click OK.
  3. Right-click Pictures.pd4, and select Rename.
  4. Change the name of file to OLD_Pictures.pd4.
  5. Start Windows Live Photo Gallery.