The leaked keys were blacklisted down the line. Online and phone activation didn't work for them anymore. They regularly updated their list of banned keys as well.

Activation became a hard requirement further down the line, I think the process got more strict during one of the service pack upgrades.

It's possible your installs were all automatically activated over the internet. I think the hardware ID and the serial number were some kind of combined identifier and if your computer(s) were part of "the good list", your fingerprint may have been whitelisted so that automatic online activation never caused a problem.

Only Home needed activation and AFAIK some OEM versions didn't either.

XP Pro/Enterprise did not.

Might have been a volume license that doesn't need activation or a cracked installation CD.

Thanks guys for the explanation :) Make sense.

Activation was delivered as an optional "feature" that would be installed alongside all the other Windows updates, unless you opted out of it (or it was already slipstreamed into the install disk).

From what I remember this was pushed around the time of SP2, but if you had a launch-day install disk and reformatted your disk that often then you probably never got as far as receiving that update.