As [Hello71][1] pointed out, it helps in such cases to read the EULA.

[Microsoft Windows Home EULA][2], Section 1.2:
> There are technological measures in this Software that are designed to prevent unlicensed use of the Software. Microsoft will use those measures to confirm you have a legally licensed copy of the Software. If you are not using a licensed copy of the Software, you are not allowed to install the Software or future Software updates. Microsoft will not collect any personally identifiable information from your Workstation Computer during this process. 

Section 4:
> You may not reverse engineer, decompile, or disassemble the Software, except and only to the extent that such activity is expressly permitted by applicable law notwithstanding this limitation.

So, yes, Windows XP Ultimate is with a pretty good possibility a violation of the EULA and there for off-limits on SE.

 [1]: http://meta.superuser.com/users/36565/hello71
 [2]: http://www.microsoft.com/windowsxp/eula/home.mspx