9. A computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: creating two indexes in a client device, the two indexes including a prefix index that indexes documents stored on the client device according to partial words that are parts of words in titles of the documents, and further including a content index that indexes the documents stored on the client device according to full words found in textual content of the documents; receiving a desktop search request including a given search term; querying the prefix index to identify two or more first documents stored on the client device, where each first document has a title that includes a word that matches the given search term or that has a prefix that matches the given search term; querying the content index to identify two or more second documents stored on the client device, where each second document has textual content that includes a full word that matches the given search term; ranking the first documents and separately ranking the second documents to identify highest ranking first documents and highest ranking second documents stored on the client device; and determining a relevance score for each of the highest ranking first documents and the highest ranking second documents; determining a first combined relevance score of the highest ranking first documents and a second combined relevance score of the highest ranking second documents; displaying data identifying the highest ranking first documents above data identifying the highest ranking second documents in a results window on the display device when the combined relevance score of the first documents exceeds the combined relevance score of the second documents; and displaying data identifying the highest ranking second documents above data identifying the highest ranking first documents in the results window on the display device when the combined relevance score of the first documents exceeds the combined relevance score of the second documents.
9. A computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: creating two indexes in a client device, the two indexes including a prefix index that indexes documents stored on the client device according to partial words that are parts of words in titles of the documents, and further including a content index that indexes the documents stored on the client device according to full words found in textual content of the documents; receiving a desktop search request including a given search term; querying the prefix index to identify two or more first documents stored on the client device, where each first document has a title that includes a word that matches the given search term or that has a prefix that matches the given search term; querying the content index to identify two or more second documents stored on the client device, where each second document has textual content that includes a full word that matches the given search term; ranking the first documents and separately ranking the second documents to identify highest ranking first documents and highest ranking second documents stored on the client device; and determining a relevance score for each of the highest ranking first documents and the highest ranking second documents; determining a first combined relevance score of the highest ranking first documents and a second combined relevance score of the highest ranking second documents; displaying data identifying the highest ranking first documents above data identifying the highest ranking second documents in a results window on the display device when the combined relevance score of the first documents exceeds the combined relevance score of the second documents; and displaying data identifying the highest ranking second documents above data identifying the highest ranking first documents in the results window on the display device when the combined relevance score of the first documents exceeds the combined relevance score of the second documents. 10. The computer storage medium of claim 9 , wherein data identifying the highest ranking first documents is displayed above data identifying the highest ranking second documents.