Its this very thing that drove me to d/l a Pro version of Xp. Yes, I did buy a home copy way back when it was released (and I can verify that if you don't believe me). I installed it on my machine.
Then I rebuilt the computer about a month later, re-installed and activated with no problems. I reformatted some after that for some reason, can't remember why, and at that point was prompted to call. I was a bit po'ed but I did. I was then told ANY time from that time forward I wanted to activate I needed to call.
I'm a bit fuzzy on this since its been so long ago, but after I activated it over the phone, it seems I had a stick of ram go bad. So I had that replaced by crucial and upon installing that stick I was AGAIN prompted to call and activate. That was enough to send me over the edge. Like I said, I can't remember exactly if it was the ram (seems like it was) or I changed video card or something, something very minor, but afterwards I was suppose to call and basically tell them what I did so I could continue to use the copy that I HAD BOUGHT!
This copy of XP was never installed on another computer. At the time it was the only pc in the house. But all the activating BS really left a bad taste in my mouth. Just because I changed my hardware around I was being treated like a kindergartner and had to ask permission to use something I paid for.