Even moving an add in card to another PCI slot is a hardware change. Some hardware is weighted more heavily than others as far as XP accepting it without complaining. You should be getting a warning that you have X number of days to activate. A call to MS is the easiest way. Answer a couple of simple questions and they give you a 50 character code to make the PC happy. Otherwise, a repair installation should make the activation screen work again. It doesn't touch your other applications or your files but does set XP back some as far as updates. I've had to call MS only once. It was a snap. Operators are waiting to take your call. Good luck.
I got a new case for an old dell 4100. I used everything from the dell in the new case except for a new video card and cd burner. When I first booted the system everything worked fine except for the cd drive. I shut the system off, moved a jumper on the drive, restarted the system and now it says I need to reactivate windows because of too much hardware change. Thats all fine and good but when the activation screen comes up its blank. I have rebuilt several systems using harddrives that have an os on them without any trouble. Any suggestions?