Web development as we know it would not exist. Microsoft would have banned competing browser engines (as is the case on iOS) and IE6 would have had 90% market share. The preferred way of doing active content would be ActiveX.
That's still a huge majority of PCs. The self-built PC has always been a bit of a market anomaly; people don't have self-built Macs and self-built cars are a truly tiny number.
And I'm working on the assumption that this would have applied decades ago, when there weren't quite so many viable alternatives to the PC and Microsoft was the terrifying market monopolist.
