Lord Voldemort stopped. He had just turned into a spirit, after trying to kill the boy, and he sensed something... strange.
He closed in, to the boy, barely a toddler, who yet had defeated him.
And he drew his - imaginary - breath. A horcrux. The boy who could defeat him - a horcrux. As long as the boy lived, he lived.
Well, that called for changed plans, of course. He wouldn't try and kill his horcrux, would he? The light would make sure the boy would survive, and then, he'd make sure he'd get his claws into him, or maybe give him just enough of a head injury that he stayed a vegetable forever.
As long as the boy lived, he lived.
After this gigantic discovery, Lord Voldemort fled to a safe place to spend the next few years.
When he did come back, he found that the boy was indeed alive. He used his blood in the ritual, and then, he started with the Cruciatus. He made sure not to kill him, just destroy his mind.
Harry James Potter was admitted into St. Mungo's a few days later, having lost his mind. He was put into the bed next to Frank and Alice Longbottom.
Success.