You probably wouldn't be able to do it since it's trademarked. If Microsoft took you to court, you'd have to prove that "x" and "box" are commonly used words in the English language.
This is an interesting article on the subject:
http://ilt.eff.org/index.php/Trademark:_Domain_Names
I think this example from the above article is an example of why you shouldn't try it:
Bad Faith Requirement of ACPA
The ACPA lists nine non-exclusive factors that courts may consider in determining whether a bad faith intent to profit exists. These include whether the domain name is part of a legal name of the defendant, whether the domain name was in use prior to registration by the defendant, and whether the mark is distinctive or famous.
#2 Ford Motor Co. v. Lapertosa, 126 F. Supp. 2d 463 (E.D. Mich. 2001). The district court held that using the famous "Ford" trademark in the domain “fordrecalls.com” demonstrated a bad faith intent to profit from the mark because “Ford” was not part of defendant’s name nor had defendant made any prior use of the domain name in connection with the bona fide offering of goods or services. Further, the “use of the word ‘recalls’ in juxtaposition with the name of a famous line of automobiles, combined with the lack of any semblance of a legitimate reason for defendant to register the domain name ‘fordrecalls’ strongly indicate[d] a bad faith intent to profit.” Id. at 466.