Bill Gates was a ruthless, cutthroat businessman who made his vast wealth by using every dirty trick in the book (and inventing a few new dirty tricks along the way) and then using Microsoft's success to effectively hold the computer industry hostage for 20 years.
He viewed any successful non-Microsoft software as a threat, even if that software was for Windows. And if that software was cross-platform he viewed it as an existential threat, since it lessened people's dependence on Microsoft.
Internet Explorer? Microsoft didn't make it. They completely missed the boat on the World Wide Web, and with the popularity of the Netscape Navigator web browser (which was available on almost every computer, from $20k SGI workstations to Macs to Windows PCs), Bill Gates & co saw a threat to Microsoft's dominance, so they rushed to get their own web browser by buying one from a company called Spyglass Software. Now, since Netscape Navigator cost money, everyone assumed Microsoft would charge for Internet Explorer, and Microsoft's contract with Spyglass Software promised to give Spyglass a cut of whatever money they made from Internet Explorer sales. So what did Microsoft do? They released Internet Explorer for free, which was something none of their competitors could do since Microsoft had such deep pockets. Spyglass Software was ruined, and so was Netscape eventually. Once Internet Explorer was available, Microsoft threatened not to sell Windows to any PC manufacturer that bundled Netscape Navigator, which would later get them in trouble with the Department of Justice and the EU.
DirectX? Began life as an OpenGL knock-off that would (Microsoft hoped) lock-in developers to Windows. Hell, Microsoft was so afraid of OpenGL (since it was cross platform and the industry standard at the time) that they offered to partner with SGI (creator of OpenGL) on a new, cross platform graphics library called FireGL. Except that Microsoft had no intention of actually releasing FireGL. They hoped working on FireGL would distract SGI from advancing OpenGL long enough to let DirectX (then called Direct3D) catch up to it, and when their plan worked Microsoft just up and abandoned FireGL.
When 3D accelerators were new (which are now called GPUs), there was a much larger number of companies developing desktop GPUs than the nVidia/AMD/Intel tri-opoly we have today, and many of them were too small to afford to create their own full OpenGL implementations. Since most PC GPUs at the time only implemented a small subset of OpenGL in hardware, Microsoft wrote a full software OpenGL implementation and then offered it to GPU companies, so those companies could just replace the parts that their GPU implemented in hardware and still have a full OpenGL driver. Once they had all spent a good deal of time doing this, Microsoft actually refused to license any of their OpenGL code for release, effectively guaranteeing that smaller GPU companies would only have support for DirectX.
Video For Windows? VFW (now called Windows Media or whatever) only came into being because Microsoft literally stole the source code to QuickTime For Windows. Both Microsoft and Intel were having a hard time getting video to play smoothly on PCs, when Apple surprised them both by releasing QuickTime For Windows, a port of their QuickTime video framework for Macintosh. QuickTime For Windows could to smooth video playback on ordinary PCs with no special hardware, and Microsoft and Intel were caught completely off guard by it. Apple had contracted out to a 3rd party company to do the Windows port of QuickTime, so what did MS do? They went to the same company and gave them a ton of money to develop Video For Windows, but an insanely short schedule, knowing full well that the company would essentially have to re-use a lot of the QuickTime For Windows source code to get the project done on time.
When Apple found out (their contract with the other company stated that Apple owned all the QuickTime For Windows source code), they went ballistic and sued Microsoft. Microsoft had been caught red-handed and knew that Apple had them by the balls. So MS settled. Remember when Microsoft "bailed out" Apple in the 90s by buying $150 million in Apple stock? Despite what the tech press reported, that's not what actually happened. The $150 million in non-voting Apple stock that Microsoft bought was part of their settlement (Apple was no longer on the verge of bankruptcy by that point, and didn't need to be bailed out). The settlement also had Microsoft agreeing to port MS Office and Internet Explorer to Macintosh.
So a lot of people my age tend to view Bill Gates' recent charities as an attempt to whitewash his reputation and, in a way, buy his way into heaven.
edit: Wow, gilded twice! Thanks!