William the Conqueror helped Britain in many ways such as the introduction of the Feudal system, but he also started things like the harrying of the North so he was not always good. I think it is up to you to decide, he certainly changed England though.
If you are referring to William I (William the conqueror), click on "William the conqueror" under "Related links" below. I think what it says in the name...he CONQUERED! obviously! Errm yerr i think he ruled for a while and then he just died?!