Why does the air flow heavily from my downstairs vents, but very weak from my upstairs vents?
Sometimes it is necessary to adjust the fins on the downstairs vents to restrict some of the airflow so that more will make it to the upper level. This is especially true in cases where there are no dampers in the ducts of the type described by BMitch in his answer.

I have a similar problem in my own two level house where I have to adjust the vent fins two times per year to set the proper upper / lower level air flow.

Another important question for you.

Have you checked the primary furnace air filter to make sure it is clean? If this has never been changed it could be restricting the air flow design of the whole heating/cooling system. I've seen these filters so clogged up with dirt and dust that it was a wonder that the furnace fan was able to circulate any air at all!!