Very good question!
In the ancient vedic texts and in Srimad Bhagavatam (composed about 5000 years ago) in particular, there is a mention of the 7 mothers for a human being who are as follows:
(1) the original mother,
(2) the wife of the teacher or spiritual master,
(3) the wife of a brahmana,
(4) the king's wife,
(5) the cow,
(6) the nurse,
(7) the earth.
From this view point, the cow is not only placed in a very high status but also is considered as a mother. Hence, like a human she is also cared for and protected.
The killing of a cow also had serious karmic reactions and hence it was/is considered a great sin. The Vedas declare that the purpose of human life is to evolve spiritually and liberate oneself from the ocean of birth and death. Taking the life of another living entity is not only causing it a great pain but also restricting their own spiritual evolution. Such killing for the sake of personal desires, is considered violence of which killing a brahamana or a cow was considered the highest of crimes. Violence causes the soul to transmigrate from one body to another (life after life), causing it to be tightly bound within the material universe. It's re-entry to the spiritual universe was very much based on (a) seeing/protecting/loving all living beings (2) devotion to God
The cow is also seen as very important from a medical and health point of view. In Vedas (and Ayurveda), the milk is considered to nourish the brain there-by helping a spiritual student assimilate deep philosophy and similar subject matters. Butter & ghee is used for cooking whereas cow dung was a natural fuel. Cow urine is an important part of ancient medicine and even in today's ayurvedic medicines. Ghee was also an important part of many of the ancient vedic ceremonies.
The cow & the bull was also seen an integral part of a just, peaceful and a non-violent soceity. Here is a statement from Srimad Bhagavatam (Canto 1 Chapter16 Vese 18):
"The bull and the cow can be protected for the good of all human society simply by the spreading of brahminical culture as the topmost perfection of all cultural affairs. By advancement of such culture, the morale of society is properly maintained, and so peace and prosperity are also attained without extraneous effort."
Purport: The bull is the emblem of the moral principle, and the cow is the representative of the earth. When the bull and the cow are in a joyful mood, it is to be understood that the people of the world are also in a joyful mood. The reason is that the bull helps production of grains in the agricultural field, and the cow delivers milk, the miracle of aggregate food values. The human society, therefore, maintains these two important animals very carefully so that they can wander everywhere in cheerfulness. But at the present moment in this age of Kali both the bull and the cow are now being slaughtered and eaten up as foodstuff by a class of men who do not know the brahminical culture. The bull and the cow can be protected for the good of all human society simply by the spreading of brahminical culture as the topmost perfection of all cultural affairs. By advancement of such culture, the morale of society is properly maintained, and so peace and prosperity are also attained without extraneous effort. When brahminical culture deteriorates, the cow and bull are mistreated, and the resultant actions are prominent by the following symptoms.
(Refer more statements here: http://vaniquotes.org/wiki/Brahminical_culture_and_cow_protection)
Finally, devout Hindus see the cow as a great object of affection for Lord Krishna. Infact, in His earthly pastimes and in His eternal pastimes in His own spiritual planet, He is seen to spend a great deal of time with the cowherd families of Vrindhavan. In books realted to Krishna, He is often described to be surrounded by millions of cows, chanting their names and taking them out for grazing everyday. He is famously known as "Govinda", the well-wisher of the cows. Hindus and eastern spiritualists, often protect the cow from this angle as well, as they want to follow in the footsteps of Lord Krishna, whom they have devoted their own lives too.
That's all I can think of for now and hope I have contributed in some way.