This for the Valencia region (sorry don't know about the others)
You can make an appointment online for your local health centre:
Instructions in English here:
http://www.timspain.com/index.php?option=com_content&view=article&id=58
Mar 3, 2010 · shaggymon
For those in Andalucia:
https://ws003.juntadeandalucia.es/pls/intersas/servicios.tramite_enlinea_citamedico
https://ws003.juntadeandalucia.es/pls/intersas/servicios.tramite_enlinea_citamedico