There are several numbers that you need to keep seperate,
mentally.
The "CD key" has only one purpose: allows you to proceed
with the CD install. It has nothing to do with activation
or registration. It tends to be of this format: 5 sets of
5 characters, separated by dashes.
Don't confuse your Windows key with your Office key, or
with any other application key, even if from Microsoft.
Windows then identifies products with "product IDs".
Example (this one is Office 2003):
90110409-6000-11D3-8CFE-0150048383C9
There's nothing secret about this ID. ANYONE with Office
2003 has this identifier. Again, it doesn't activate or
register with MS.
Activation is required for Windows, and it's also
required for Office. This "Activatiion" number comes from
Microsoft, not HP. You need to activate either online, or
over the phone.
Registering is not required, but it will happen if you're
online and go to Windows Update automatically, whether
you like it or not. Registration happens secretly without
you knowing - but you know now.