Activation is set by a number generated by a mix of hardware id's taken from your system. If those numbers change significantly (how significantly? Who knows?) then activation is tripped and you have 30 days to figure it out. I just changed motherboards and processors, and it wouldn't let me activate, but there's a number you call, and you read off this long number to them, and they read back another long number to you which you type in a dialog box, a pretty simple process really, and they say OK and you are activated. With a change in hd it is quite possible you won't even have to do anything, but worst case you have to read a lot of numbers.
Thanks for the Reply! I just don't like the feeling that "big brother" is watching. I have an oem copy came with my computer. Redid the system using win me and 2000 dual boot. Finally installed xp (kicking & screaming) my kids really like it. Now that we've decided to keep it we have to deal with "the power that is". "Bill Gates may I?" just isn't a phrase I want to use.
If this doesn't work you will have yo reactivate with MS.
XP - Re-Activation
Once you activate your copy of Windows XP operating system, a file gets created/updated in windows/system32 directory. It is called wpa.dbl file. Take a backup of this file and when you have reinstall the OS just copy it back to the same directory.