In "I Robot, You Jane" when Giles asks Jenny if she was a witch, she replied, "No, I don't have that kind of power. Technopagan is the term." So based on that statement, I would say that the origin of magickal power is still a bit ambiguous: she could mean she wasn't born with that kind of power or that she hadn't yet acquired that much power.
I think magick is something that is both learned and that you can have an innate ability for - just like for most skills (music, sports, etc...) Knowing Jenny grew up in a culture where magick must have been around a lot and that as an adult she still said she hadn't that much power, on top of the fact that even Giles who raised a demon for fun was weary about performing the ritual again shows how much natural affinity Willow had for magick. She obviously channeled something when performing the ritual: a spirit, Jenny, or it could have just been her own natural ability awakening.
So in general, I agree with everyone here that magick is something that you can have a natural talent for, but it is also a skill you can build. So to answer the OP, I think the restoration spell was the moment where Willow's power and potential was first tapped and it is significant that it occurred with a spell that even Giles was hesitant to perform. But I think any spell that was Willow's first would have relieved her natural affinity for magick.