Isn't a monopoly when you sell a product no one else is able to (natural monopoly) or allowed to (legislated monopoly) sell? What's preventing people from usin

Isn't a monopoly when you sell a product no one else is able to (natural monopoly) or allowed to (legislated monopoly) sell? What's preventing people from using other search engines? Microsoft's monopoly was based on exclusive contracts with PC makers, which made it extremely difficult to buy a PC without paying the "Microsoft tax".