There is a very thorough article written by Marvin Harris, which can be accessed at http://sociology101.net/readings/Indias-sacred-cow.pdf. This should supply you with much-needed information as it provides the history of the reverence for the cow and much explanation. Here are some early points from this article:
Cow worship evolved in India in the last 3.000 years as the Hindu religion took hold and developed. The Vedas from the First Millennium B.C. some of whom did not revere the cow and ate beef, while others held a taboo on the slaughter of cows. Then, in 200 A.D. the Brahman priesthood urged the population to revere the cow and prohibited them from eating beef. Furthermore, from this history of the sacred cow, it can deduced that the prohibition against eating cows, bulls, and oxen--all beef--has a sound logic to it because the bulls, oxen, and especially the cow, who can produce bulls and oxen, are connected not just to the spiritual rebellion against invading Islamists who do eat beef, but also to agriculture.
The peasant whose cow dies is not only crying over a spiritual loss but over the loss of his farm as well.
Without the animals to pull the plows, farmers cannot sow their fields before the monsoon season begins.
Another reason for making the cow sacred and its meat prohibitive is that during droughts, farmers turned their cows loose so that they could scavenge along roads, in the towns, etc. When the drought ended, the farmers would reclaim their animals and use them for agricultural purposes, and they would reclaim any cow that produced a calf or was lactating. Thus, Morris observes,
The prohibition against beef consumption is a form of disaster insurance for all India.
Still, a further reason for prohibiting the slaughter of cows is the fact that the milk of Hindu cows is extremely important for the production of many foods that the Indians consume. In addition, the cow is useful in the production of panchagavya, which is a fertilizer and pesticide, both.
I think that the origins of the sacred cow in India comes from the Hindu faith. Hinduism embedded cows as an essential component in its faith. This can be seen in distinct ways. In one of Lord Vishnu's avatar, as Kurma, he assumed the role of a tortoise that provided the base for the churning of the celestial oceans. Out of this churning came forth the powerful cow of Kamadhenu. As Kamadhenu emerges, the Hindu faith established that the cow was the embodiment of "purity and non-erotic fertility, ... sacrificing and motherly nature, [and] sustenance of human life." Adding to this is the idea that within the Kamadhenu form of life exists millions of deities. Thus, to revere the cow is to pay homage to multiple aspects of the divine. This enshrinement is further seen in another one of Lord Vishnu's avatar, when he appears in the form of Lord Krishna and is a herder and protectorate of cows. From this, cows were seen as an essential component of the Hindu faith. Lord Shiva's constant protector and guardian is Nandi, the bull. When Lord Shiva grants immortal life to Nandi, he makes him a bull that will serve as his vahana, or vehicle, and always remain by his side. The cow was seen as a force of immortality.
It is in this regard where cows are sacred in India. They are viewed as a direct representation of the divine. To do offense to the cow is to offend the divine cosmological construction of the universe. The Vedic scriptures all affirm the divine emanation of the cow. The cultural appreciation of the cow takes different forms, but it comes from this divine root. It is in this regard where the reverence for cows began.