In the US, there are two ways to infringe trademarks:
1) Using the trademark in a way likely to cause confusion - generally has to be in the same line of business.
2) Trademark dilution - using a famous, distinctive brand and making it a generic term.
Entrepreneur, Apple and Windows aren't distinctive in their own rights (the logos might be, however), but some names certainly are.
Some seminal examples of this (in Australia at least) include "McDonald's" (pretty much impossible to brand any good or service, no matter its class, as such) and "Nike" (the shoe manufacturer prevented the manufacture of "Nike" perfume).
Trademarks are usually registered in one or several international classes (there are 45 of them), but there's a cost per class, so most companies don't try to cover the whole spectrum:
So, you can have a "Widgets" trademark for class 9 (Electrical and scientific apparatus), and another company can own the "Widgets" trademark for class 29 (Toys and sporting goods).
Some companies do register their trademark across the whole spectrum, like Coca-Cola, and probably Nike too.
Note that pharmaceuticals are a whole different story, as there are so many of them that you can have similar-sounding ones as long as they do something very different.
Entrepreneur, Apple and Windows aren't distinctive in their own rights (the logos might be, however), but some names certainly are.