I just finished a repair for a customer where i had to image his failing HDD to another one to rescue what i could...
Imagine my suprise when i got the activation notice as well.
A hHDD is not a major hardware change yet i still got the notice.
I clicked on activate and chose activate but do not register and it worked fine.
If you are still gettting the prompt after trying this contact the Mothership...if you're legit it's a painless process...
Good Luck
XP Pro BTW with all updates.