I got a cracked copy of 3ds Max at a LAN party (back when it was "Discreet 3dsmax"), and immediately dragged dozens of cubes, spheres, and cones into the scene.

Then I closed it for a year. Opened it up again one day, followed a box-modeling tutorial (from the documentation PDF linked in the Help menu!), and I was hooked. Spline modeling, rigging, walk cycles, texturing, lighting experiments, every spare minute for the rest of high school.

I still remember the whole-body panic of accidentally turning on "adaptive degradation", which replaced all meshes with their bounding cubes when rotating the viewport camera, and thinking I had broken my video card.