said by cdru :

Even then, it's not 100% clear on the trademarking of Windows. Back a few years a company called Linspire came out with a Linux distribution that ran some Windows applications called Lindows. Microsoft sued because the trademarks were sufficiently similar that it could cause confusion to consumers. While the case was ultimately settled out of court before all appeals were exhausted, it was quite a ways down the path of Microsoft losing the case because Microsoft had used the term "windows" in describing the GUI prior to Windows (the product) from ever being released. Apple and Xerox had also implemented the idea and used the terminology prior to Microsoft telemarketing it as well. It would be similar to me trademarking the name Phones and then suing any company that used the derivative of that.

Trademarks are used to product product name recognition in the market place. Multiple companies can own the same or similar trademark as long as the use of the trademark is in different areas and would not be commonly confused by the lay person. My company has a trade mark of "Do it Best" (the hardware store). Nike has the trade mark "Just Do It". They are fairly similar, but it's not likely someone would confuse a hardware store and a sports clothing company.