As the head of Windows product development at Microsoft, Julie Larson-Green is responsible for a piece of software used by some 1.3 billion people worldwide. She’s also the person leading the campaign to introduce as many of those people as possible to Windows 8, the dramatic redesign of the iconic operating system that must succeed if Microsoft is to keep pace with a computing industry now shaped more by phones and tablets than desktop PCs.

Windows 8 throws out design features familiar to Windows users since 1995, swapping in simpler, bolder interfaces designed to be operated using a touch screen. The release of the Surface, a device somewhere between a tablet and laptop, also sees Microsoft break its tradition of leaving the building of hardware to other companies.

Larson-Green took over the role a few weeks ago, after Microsoft veteran Steven Sinofsky left amid rumors of personal disputes with other Microsoft executives. However, Larson-Green has long been a senior figure inside the Windows division and even took the lead on drawing up the first design brief for Windows 8. An expert in technical design, she also led the introduction of the novel, much-copied “ribbon” interface for Microsoft Office, widely acknowledged as a major improvement in usability.

Larson-Green met last week with Tom Simonite at Microsoft’s campus in Redmond, Washington.

Why was it necessary to make such broad changes in Windows 8?

When Windows was first created 25 years ago, the assumptions about the world and what computing could do and how people were going to use it were completely different. It was at a desk, with a monitor. Before Windows 8 the goal was to launch into a window, and then you put that window away and you got another one. But with Windows 8, all the different things that you might want to do are there at a glance with the Live Tiles. Instead of having to find many little rocks to look underneath, you see a kind of dashboard of everything that’s going on and everything you care about all at once. It puts you closer to what you’re trying to get done.

Windows 8 is clearly designed with touch in mind, and many new Windows 8 PCs have touch screens. Why is touch so important?

It’s a very natural way to interact. If you get a laptop with a touch screen, your brain clicks in and you just start touching what makes it faster for you. You’ll use the mouse and keyboard, but even on the regular desktop you’ll find yourself reaching up doing the things that are faster than moving the mouse and moving the mouse around. It’s not like using the mouse, which is more like puppeteering than direct manipulation.

In the future, are all PCs going to have touch screens?

For cost considerations there might always be some computers without touch, but I believe that the vast majority will. We’re seeing that the computers with touch are the fastest-selling right now. I can’t imagine a computer without touch anymore. Once you’ve experienced it, it’s really hard to go back.

Did you take that approach in Windows 8 as a response to the popularity of mobile devices running iOS and Android?

We started planning Windows 8 in June of 2009, before we shipped Windows 7, and the iPad was only a rumor at that point. I only saw the iPad after we had this design ready to go. We were excited. A lot of things they were doing about mobile and touch were similar to what we’d been thinking. We [also] had differences. We wanted not just static icons on the desktop but Live Tiles to be a dashboard for your life; we wanted you to be able to do things in context and share across apps; we believed that multitasking is important and that people can do two things at one time.

Can touch coexist with a keyboard and mouse interface? Some people have said it doesn’t feel right to have both the newer, touch-centric elements and the old-style desktop in Windows 8.

It was a very definite choice to have both environments. A finger’s never going to replace the precision of a mouse. It’s always going to be easier to type on a keyboard than it is on glass. We didn’t want you to have to make a choice. Some people have said that it’s jarring, but over time we don’t hear that. It’s just getting used to something that’s different. Nothing was homogenous to start with, when you were in the browser it looked different than when you were in Excel.

I wonder if you’re experiencing a little déjà vu, after previously leading a radical change to the interface for Office that initially met with complaints.

Yes! A lot of it is familiar. Some people who review it for a shorter period of time may not feel how rich it really is. We’re going for the over time impression rather than the first 20 minutes out of the box. We’ve found that the more invested you were in the old way, the more difficult the transition is, which is unfortunate because we first hear about everything in the tech press. Those are the ones that we knew up front are going to have the most challenge.

How long does it take people to adjust?

Two days to two weeks is what we used to say in Office, and it’s similar in Windows 8. We do a “living with Windows” program where we watched people over a series of months in their household. A lot of people don’t have trouble up front.

What data do you have on how people buying Windows 8 are reacting?

When you sign in to your Windows PC, one of the things you get asked is whether you’ll be part of our customer experience improvement program, and if you will, then you’re sending some data to us. Everyone gets asked that. We get terabytes and terabytes of data every day, and we can’t possibly use it all. So far we’re seeing very encouraging things. Over 90 percent of customers, from our data, use the Charms and find the Start screen all in the first session. Even if you’re a desktop user, over time there’s a cutover point around six weeks where you start using the new things more than the things you’re familiar with.

Microsoft has chosen to make its own hardware for Windows 8 with the Surface tablets. Why not leave that to the equipment manufacturers, as you’ve done in the past?

It was a way to test our hypothesis of a new way of working. It takes time for individuals to adjust, but it also takes time for the industry to adjust to new things—all the complicated things about the supply chain and issues like what sizes of glass gets cut. Surface is our vision of what a stage for Windows 8 should look like, to help show consumers and the industry our point of view on what near perfect hardware would look like. We believe in Surface as a long-term product, but we know that partners will have other innovations and ideas. One of the things that’s always been nice about Windows is choice—you’re not locked into one size, one shape, one color, one version.

Your predecessor, Steven Sinofsky, was widely credited with driving Microsoft to create Windows 8 through sheer force of will. Is that true?

Steven is an amazing leader and an amazing brain and an amazing person, but one person can’t do everything. It’s really about the team that we created and the culture that we created for innovation.

What changes now that you’re in charge?

Not a whole lot. I’ve worked directly with Steven for seven years but known him for the whole 20 years I’ve been at Microsoft. We think a lot the same about what the role of Windows is in society, what computing looks like, and getting people on board with that point of view.

Now that Windows 8 has been released, what are you and your team doing now?

We didn’t really slow down. There are always new technologies to think about that can be helpful to people.

Read more about Microsoft’s efforts to track users’ reaction to Windows 8: Microsoft Has Been Watching and It Says You’re Getting Used to Windows 8.