Regustration in Virtuemart2

[cespal]

Since I do not find the way of doing that a user buys without to be registering in virtuemart, I have decided that they should have to register. To be able to do a simple task, I want to retouch the fields and others ... but I return to find another arduous task. If I deactivate or activate options of the buyer's field in Virtuemart 2 it does not even case to me. It works when I open a module account maintenance and give him a position, but not in the form of general login. Why? Wherefrom it extracts the form of login? At the Virtuemart not, because if I remove options it doesn't work. Can you help me? Use Joomla 2.5

Thank you very much