I really don't think Microsoft stole the concept of Apple OS 9 and used it in Windows XP. I think it is very easy to believe this allegation since both operating systems are similar in some ways. Also, if you consider the fact that Apple had released its operating system some years before Microsoft released Windows XP, you might think Microsoft stole the concept of Apple OS 9.
Apple is known for its innovative products, and since there are other competitors, they would definitely want to try and copy what Apple is doing. As far as I'm concerned, I don't see this as theft since Apple couldn't argue that Microsoft stole the source code of its operating system.
The fact that someone has already tried a new thing does not mean others cannot try and produce something similar to it. However, a similar decision was reached by the court when Apple sued Microsoft for copyright infringement.