Sati – How this Hindu sacrificial practice in India started and still goes on?

Growing up in Punjab, I had heard so many times about Sati practice in Hinduism and learned that Sikh Gurus wrote against the practice. The ritual sounded cruel to me as a kid and I kind of accepted it as a Hindu thing but never really understood as to why, how and when it originated within Hinduism till today? If you are in a same position as me, here is some info:
http://www.dalsabzi.com/Wisdom_Scrip/sati_and_parvati.htm

What bothers me more is that even within the currents laws of democratic India people's lives are affected. I mean this is year 2006 and it still goes on.
http://timesofindia.indiatimes.com/articleshow/1388185.cms
http://www.indianchild.com/sati_in_india.htm

And, they claim Sikhs are just Hindus! I haven't even started on the issue and laws which affect Scheduled castes (SC) and Scheduled Tribes (ST), against which our Gurus wrote as well.