Well, in order to live in Spain for more than 183 days a year you need the equivalent of the National Insurance number in Spain this is called the NIE (Numero de Identidad de Extranjeros).
You need the NIE to do the following.
• Apply for a business permit
• Apply for a mortgage/loan
• Buy/sell property in Spain
• Buy a vehicle
• Employment
• Inheritance of assets in Spain
• Insure Property
• Pay taxes
• Sign onto the national Social Security Health plan
It seams to be a simple process to get one, fill in a form, go to the local police station (in Spain) to hand in the form (with photocopy of the form, and a copy of your passport), then wait about 10 days and pick up a letter with your NIE.
The only drawbacks to this simple process are as follows
1. The form is all in Spanish, and so are all the notes.
2. The police stations are also used for renewal of DNI's Passports and other items (so a lot of people use them)
3. They are normally only open in the morning.
To enable this process to go as smoothly as possible, obtain the form by going to this site http://www.mir.es then entering NIE in the search box and pressing enter, in the results you should see ex_14.pdf, this is the form, you can fill it in on-line and then print it (imprimer), or just print it and fill it in manually. When the upload a new form the link changes so I am not linking to it directly.
Once filled in, dated and signed make a copy of this and your passport and get to the local police station early in the morning (some operate a ticket system so when you have the ticket you can go for a coffee etc, calculate about 5-10 mins for the meeting. You will be given the copy back, stamped with the official stamp, this is required to collect your NIE when it arrives back in the station.
To collect your number, do not queue up, pop inside and simply collect the letter by showing your stamped form to an official. (this is usually 10 days after you submitted the application.)