This story is part of CNBC Make It's The Moment series, where highly successful people reveal the critical moment that changed the trajectory of their lives and careers, discussing what drove them to make the leap into the unknown.

Bill Gates put off choosing Microsoft over college for as long as he could.

As a 20-year-old Harvard University undergraduate, Gates had such "a great time" in college that he struggled intensely with the decision to leave and start a software company. After co-launching Microsoft with his high school friend Paul Allen in 1975, Gates still wanted to return to Harvard to complete his degree, he says.

The decision to leave came with some urgency. In high school, Gates and Allen predicted that microprocessors — small computer chips — would eventually turn bulky, expensive computers into small, affordable, personal machines accessible to everyone.

Gates didn't think the technology was ready, though — until 1974, when Allen "burst into my room" carrying the latest issue of Popular Electronics, he writes in his new memoir, "Source Code," which published last week.

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The magazine's cover featured the "world's first minicomputer kit to rival commercial models," a computer made by Micro Instrumentation and Telemetry Systems (MITS) called the Altair 8800. Quickly, Allen and Gates believed they could be at the forefront of a new industry: creating software for the computers that'd eventually end up in nearly every American household.

If they didn't act quickly, someone else might beat them to the punch.

But Gates loved Harvard's intellectually rigorous atmosphere, where he could learn deeply about a wide array of topics, he says. He spent much of Microsoft's first year torn between Albuquerque, New Mexico, where Microsoft was initially based, and his dorm room, he writes.

He tried running Microsoft from afar, returning to Harvard for two more semesters in 1976. He even tried to convince early Microsoft programmer Ric Weiland, another high school friend, to "take charge of things," so he could finish his studies, he says.

Instead, Weiland left for graduate school, then returned to Microsoft for a spell before ultimately resigning and moving to Los Angeles, Gates writes. (Weiland died in 2006, at age 53.)

"Even Ric wasn't going to pull things together with the intensity that I knew we needed to stay in front," says Gates, who never earned his degree, serving instead as Microsoft's first CEO until stepping down in 2000.

By then, the company revolutionized the computer industry and turned both co-founders into billionaires. Today, Microsoft is worth more than $3 trillion.

Here, Gates discusses why he was reluctant to leave college, the moment he realized Microsoft could no longer wait and why he still doesn't recommend dropping out despite his own success.

CNBC Make It: Why did that magazine cover spur you and Allen to start Microsoft so quickly? Did leaving Harvard feel like a big risk?

Gates: The key moment was when the Altair showed up. Until then, we felt like, "Hey, this is going to happen and we'll figure out our timing and what kind of company to do."

The panic about, "God, it's happening without us," was when Popular Electronics had the kit computer [on its cover]. Little did we know, [MITS] basically hadn't assembled any of them.

The idea of risk really didn't feature. If I left college and failed at that point, I could've gone back. And I wasn't borrowing money. I had money from the programming stuff that we'd done [in high school], and software companies aren't very capital intensive.

But I enjoyed Harvard. I enjoyed the classes, including some that I just sat in on: psychology, economics, history courses. I loved having smart people around. We could sit and talk late into the night about very interesting things.

I wasn't super social, but I had enough friends that I felt very comfortable. I was perfectly in sort of a nerdish lifestyle, learning a lot. So I did hesitate for that year.

I had to give in to the inevitable, and give up school and, of course, never go back.

If you'd finished your degree before founding Microsoft, would you have missed your window of opportunity?

Maybe. We really were there at the very, very beginning.

We went to these computer clubs, and even those people [didn't see that software development] would totally become the computer industry, and that the variety of software you'd need would be huge. Even then, they were kind of narrowly looking at, "Oh, can we play games on this thing?"

It was good to be early. We learned a lot [about how to run a business], and we moved up to Seattle, where it was easier for us to hire [the best software engineers].

What was your best-case scenario, initially?

We were crazy ambitious, saying things like: "A computer on every desk and in every home." We didn't see any type of software [we wouldn't make] — database, word processing, spreadsheet — unless it was a very niche thing.

You also have to keep your expenses under control. Whenever I would do yearly retreats, I would have everybody — in private — estimate what our revenues would be, say, five years hence. Weirdly, Steve [Ballmer, Microsoft's first business manager and eventual CEO] and I would always be on the low end.

We were so afraid that everybody around the table would expect to get a lot of head count and expense, and we were so desirous of not running out of money.

But, then again, I was the madman who said, "We have to do all of these new products, and we have to get them done really quickly."

What's your advice now to someone considering dropping out of college to start a business?

You know, I'm not some person who promotes dropping out of college. I'm a huge fan of a broad set of knowledge, and I think it's an exceptional case where the urgency is such that you interrupt those college years to go do something else.

I got to use almost all of the knowledge that I acquired [in college]. I was glad that I had the curiosity to take all of those different courses. So I'm always encouraging people to learn a broad set of things.

This interview has been edited and condensed for clarity.

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