$ perror 12
OS error code 12: Not enough space
The SELECT need some tmp space to gather the answer; it ran out of disk space. Suggest you find some files to remove. On Windows, emptying the Recycle bin is often a quick way to do so. If you can't free up some space, you are in big trouble; MySQL tricks like OPTIMIZE cannot be done without some space.
Another possible cause... Where is the tmp directory for MySQL? Sometimes a system has two partitions, a small one for the OS, a big one for the rest. But tmpdir may be pointing to the small one. Change my.cnf and restart mysqld.
Your current query:
SELECT idArticle, surtitreArticle, titreArticle
FROM Articles, ArtRub, ArtSousRub
WHERE idArticle = ArtRub.FK_idArticle
AND idArticle = ArtSousRub.FK_idArticle
AND FK_idRubrique='2'
AND FK_idSousRubrique='29'
AND magArticle='Le Nouvel Observateur'
ORDER BY dateArticle DESC
LIMIT 0, 1;
SELECT idRubrique, nomRubrique, typeRubrique
FROM Rubriques, ArtRub
WHERE FK_idArticle = '1242'
AND FK_idRubrique = idRubrique
ORDER BY idRubrique;
It is hard to tell since there are not table qualifiers, but probably you are not accidentally doing a "cross join" -- something that can take an immense amount of space.