Vista - Indexing service error

Does anyone know what might cause this repeated failure message for the
Indexing Service?:

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The indexing service detected that its index of metadata associated with
documents is corrupted. These documents will not be retrieved correctly
during a search. To correct this, the indexing service will rebuild its
indexes by scanning the required folders and processing all documents again.
The documents themselves are not affected, but a search for these documents
will not be successful until the scan is complete.
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What might be causing the metadata corruption and what settings (if any)
could be optimized to prevent this from recurring?

Thanks,
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V2