Microsoft has turned 40-years-old today, making it among the oldest tech companies in a time when startups are all the rage. One of the biggest brands by market value, Microsoft makes products that are used by billions of people worldwide.

Windows was the breakthrough product for Microsoft, a software that reduced the dominance of the hardware in the computer industry. On Microsoft 40th anniversary, let us take a look at how Windows has changed since it was unveiled in 1985...

The first version of Windows was basically a simple layer on top of MS-DOS, the character based operating system that ran most PCs then. It was not widely used.

This was the first operating system to run Microsoft Word and Excel. It also caused Apple to file a "look and feel" lawsuit against Microsoft for ripping off some elements of the Macintosh and Lisa. Apple did not win that case.

Windows 3.0 did a lot more to hide the underlying MS-DOS guts with graphical icons like the Program Manager. Not surprisingly, it became the first really popular version of Win.

This is the version of Windows that really made it the standard on most IBM-compatible PCs back in the early 1990s. It's also the last version that kind of looked like MS-DOS.

Windows 95 was a big break with the past, emphasizing the desktop and introducing icons for almost everything. Internet Explorer, the Recycle Bin, and the Start button were all introduced here. The basic design for Windows hardly changed from this up until Windows 8, which came out in 2012.

It's hard to understate how big Windows 95 was. Jay Leno joined Bill Gates on stage at the launch event. The Rolling Stones song "Start Me Up" was used in ads. People lined up to buy it — like they do with the iPhone today.

If it ain't broke, don't fix it. This update looked and worked just like Windows 95, but was more stable and had a few extra features.

This was basically Windows 98 with a few new consumer features. It didn't sell very well. Most people waited for XP, which was known to be coming soon.

Around the same time, Microsoft began putting out its first versions of Windows for mobile devices. It was based on a completely different underlying technology called Windows CE.

Microsoft tried to keep a lot of Windows branding front and center, like the Windows logo and the Start button.

In 2001, we got Windows XP. It was the first big update since Windows 95, and the first consumer version of Windows to break completely from MS-DOS and use the same underlying technology as Windows NT — the more stable and powerful version of Windows that had been used on servers and "workstation" PCs.

Meanwhile, Microsoft was still putting out new versions of Windows Mobile, trying to compete with the BlackBerry and Nokia phones, which were dominant at the time. It had no idea the iPhone was about to come along and change everything.

Windows Vista took Microsoft five years to build, and there were a lot of missteps and resets along the way. It was not a hit, and former CEO Steve Ballmer has said that the time and effort the company spent on Vista — instead of pursuing new opportunities in mobile — was one of his biggest regrets.

This is where things really started to go south. Windows Mobile 6 was a continuation of Microsoft's five-year-old mobile platform, and came out in February 2007 — right after the first iPhone was announced.

It was still built for use with a stylus and keyboard, not finger touch, and it looked ridiculously rudimentary next to the iPhone. But Microsoft in public downplayed the threat, with Steve Ballmer doubting anybody would ever pay $500 for a smartphone.

Fortunately for Microsoft, it was able to recover from the Vista debacle on the desktop. Windows 7, which came out in 2009, was basically a polish-up of Vista, but much more stable.

It's still very popular in businesses. In fact, one of the big challenges with Windows 10 will be getting enterprises to upgrade from Windows 7.

At the same time, Microsoft knew it needed an answer to the iPhone. In 2010, it released the first version of Windows Phone. It was a clean break with Windows Mobile, with a totally new finger-friendly design and a bunch of new features.

But the iPhone and Android were already growing by leaps and bounds, and the new platform did not take off.

The iPad came out in 2010, and its early success spooked Microsoft into another misstep. Instead of continuing to adapt on 30 years of of desktop success, Microsoft took a lot of the concepts from the unsuccessful Windows Phone operating system and brought them over to the main version of Windows.

The goal was to make an operating system that functioned equally well on touch-screen tablets, to compete against the iPad and regular PCs. The main interface had no start button, huge colorful boxes instead of icons, and changed how basic tasks were done.

...but it was buried under the new interface.As a result, a lot of traditional Windows users were confused or put off, and Windows 8 never took off. Instead of coming up with an operating system that was great on both PCs and tablets, Microsoft had built one that was great on neither.

PC sales have plunged more than 12% in the two years after Windows 8 was released, although this can't be blamed entirely on Windows 8 — people have more choices now, and the overall economy has been weak.

Microsoft also updated its mobile platform at the same time as it released Windows 8. It looked the same on top, but there was a lot of new work under the covers: basically, Windows Phone was now based on the same underlying technology (Windows NT) as Microsoft's PC and server operating systems.

Last year, Microsoft updated both its PC/tablet and mobile operating systems, and began to bring them closer together. It also used Windows 8.1 to reverse some of the big changes it had made with Windows 8, like returning the Start menu.

Windows 10 is expected to continue in this vein, bringing the PC, tablet, and phone platforms even closer together and making the traditional Windows desktop more easily available if you're using a keyboard and mouse.