To get to the Options d'indexation:
Start --> Panneau de configuration --> Options d'indexation
See Change advanced indexing options for more information.
If you cliquez sur the Advanced button in Options d'indexation and allez dans the File Types tab, you will get a list of type de fichiers and the way they are indexed. For the type de fichiers you want, vous pouvez specify that you want the file contents indexed, and not just the file properties.
Or vous pouvez just do a normal search, and after the search is finished vous pouvez cliquez sur the "File Contents" button under the "Search again in" field (which is located after the end of the search results list, if you scroll to the bottom).
Based on this page, the "File Contents" option won't always show up - only when the folder being searched is not marked for file content indexing; in that case, file contents are supposedly searched automatically, without having to specify this option explicitly.