Question: Is there an official Windows XP registry reference online somewehere? I see there's a reference for [Win 2000](http://technet.microsoft.com/en-us/library/cc974061.aspx) and [Win 2003](http://technet.microsoft.com/en-us/library/cc778196(v=ws.10).aspx) servers on MS technet, but I can't seem to find one for XP. Yes, I know that Win 2000 reference would be good enough (most things will be the same in XP anyway), but if there's an explicit reference for XP, I'd like to take a look at it.

EDIT: I didn't mean API reference. I'm looking for a reference with descriptions of registry keys, possible values, etc.

Answer: 
The Windows 2000 Resource Kit comes with a help file called "Registry Reference" (regentry.chm). If you have access to this older Resource Kit, this file is very helpful.
The registry is just a means of storing data. Large portions of it are inherently undocumented and subject to change. The registry was not designed to be closely defined, so there cannot be closely defined or complete documentation for it.

For specific situations (like "what values are allowed in `HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config\UpdateInterval`") you may find vendor documentation (in the MSDN in this case).