Besides being a smart guy, ....
He's not a smart guy, but he is a bully.
He bought QDOS for $30,000 and as part of the contract, agreed to employ the creator of QDOS as a consultant then Welshed on the deal.
Because Gates wasn't smart enough to understand QDOS, and because none of the idiots he hired to mess with it understood it, there were a lot of idiosyncracies introduced by Gates into MSDOS.
Those idiosyncracies were called "virus."
For example, if you told the system to write 0 bytes to a file, then wherever the file pointer was (on the floppy disk, hard disk or hard drive) it would truncate that file.
So, I could write a few bytes of code telling the system that whenever you pressed a certain key on your keyboard to write 0 bytes to the file, send it to you in an email, opening the email would download it on your computer, and it would screw up all the files on your discs after a few hours of use.
Windows? Gates didn't invent that. He bought it. Specifically, he bought it from the Software Group.
I was using Windows before Windows existed. It was a powerful software program called Enable that ran on QDOS.
So, I'm at TRADOC HQ and I just finished up the TOE for Division '86 as part of the AirLand Battle 2000 doctrine.
I open up a WordPerfect-style doc to use as my target doc.
Then I open up "the Blurb doc", you know, security classifications, disclaimers, legal notices, etc etc, and then I open up the doc that has the text I want to send to the target doc.
See, in MSDOS, you could only have one file open at a time.
Then I open up a Lotus-style spread sheet with the TOE, another spreadsheet with the TDA, and another with all the cost estimates and financial stuff.
See, in MSDOS, you could only have one program running at a time.
Then I open up a DBA database that has all the address info for all the major commands.
Then I'd open up the telecom program.
I'd put the telephone bit into the cradle for the modem, run the mail-merge and send all the stuff from the other docs into the target doc and presto! It starts trolling through the phone numbers of all the major commands and transmits the doc through the modem.
Anyway, Gates bought it, took it off the market for 6-7 years or so and it was reincarnated as Windows, which had a lot of problems because Gates isn't nearly as smart as you think he is.