Some thoughts on the originality of Microsoft product design...

Microsoft BASIC
The programming language BASIC was put in the public domain ten years before Bill Gates and Paul Allen thought about selling a BASIC interpreter for the Altair home computer. The success of Microsoft is based on its interpreter business of the 70's, because it put Gates in touch with IBM.
Multiplan, Excel
Dan Bricklin asked Microsoft to sell his revolutionary spreadsheet software called Visicalc, the killer app that made Apple II the best selling home computer of its time. Microsoft denied any deal, copied the software and entered the application software business with Multiplan. Multiplan later became Excel.
MS-DOS
Bill Gates sold to IBM an operating system that he didn't have. And there wasn't enough time for development. So he went shopping and got QDOS. A good deal? Well, rumor has it that MS-DOS did not only copy ideas, but also code from CP/M86.
DoubleSpace, DriveSpace
The killer feature for MS-DOS 5.0 was the ability to enable compression for whole disk partitions. This feature came bundled with a couple of problems which were responsible for a lot of bad karma in form of data lossage at that time. The compression algorithm led to two MS-DOS updates: Microsoft did not only steal it from Stac Electronics' compression product Stacker, but they also got caught. Stac Electronics received US$120,000,000 in damages.
Windows
Microsoft did neither invent the WIMP GUI (graphical user interface using windows, icons, menus|mouse and pointer), nor did Microsoft make GUIs popular. The WIMP principles were developed at Xerox PARC, the Palo Alto Research Center, and both Steve Jobs and Bill Gates took a demo and copied everything they were shown. Apple was in production with Lisa, which had a GUI, when Microsoft still tried to add stuff as hard disk and folder support to MS-DOS.
But even if you limit your world to Intel CPUs, Microsoft didn't have the GUI pole position. Dan Bricklin got his second lesson, this time in FUD (fear, uncertainty, doubt). The PC-based GUI VisiOn was ready-to-market but lost every chance when Microsoft announced Windows as a direct reaction to VisiOn.
Nevermind that it took Microsoft another two years to put Windows on the market, and another four years to render it usable. The Microsoft operating system base, solely built on MS-DOS, gave them an advantage big enough to beat every rival product.
Windows NT
The father of Windows NT was the lead developer for VMS and OpenVMS - what made the difference between Win3.1 and WinNT were VMS concepts.
Internet Explorer
The Internet was thought up in the 60's. It was built in the 70's from Arpanet. It grew worldwide in the 80's - and everyone who touched it back then, felt the magic and anticipated that a new communication media was becoming.
Windows 95 was the first Windows version that supported a dial-up connection to the Internet without the need for additional software.
Only after nobody could deny the success of the World Wide Web (and Netscape) anymore, Microsoft decided that they've always wanted to integrate Windows and the Internet, and they got a licence of the Spyglass browser, which they called Internet Explorer.

Almost forgotten...
Wordstar -> Word Perfect -> Microsoft Word
subLogic Flight Simulator -> Microsoft Flight Simulator