If you are planning to rent from Santiago de Compostela, you can choose from the big names like Hertz and Avis. Just recently (as far as I know) EasyCar also serves Santiago de Compostela. They are supposed to be a bit cheaper, but with all the offers that always exists, you would have to check.
For some more information on car rental in Santiago, have a look here:
http://www.santiago-today.com/rent-a-ca ... ostela.cfm
Have a look at EasyCar here:
http://www.easycar.es
... or by clicking this link:
http://ad.zanox.com/ppc/?3515875C823770166T
This link takes you to the EasyCar website as well; the only difference is that I will get a kick-back if you ever make a reservation.
I hope this helps a bit.
Greetings from Santiago de Compostela,
Ivar