Creating N fields with N 'subfields' I'd like to create a content type "Enterprise" where user can define N services and, for every service, N tasks. I have already created the other fields for this content-type (name, description, ..., easy fields) but I don't know how to start with the services and tasks.

I'd like to be able to create services and tasks in the same form. What would be the best way to do it?
In case of use d6, you can use Flexifield, then after you install you should create a content type that should represent your subfields, and in your main content type you should create a new field, with Flexifield type, and then, define the it's a number of values of the field, and then, in the creation node form of your main content type, you`ll have a field (kind of group), that will have multiple subfields, and depending of the number of fields defined, you should have multiple "groups of subfields".