"it says it's activated" when did this occur? When you reinstalled XP after replacing the motherboard or are you meaning that you assume it's activated because you only hooked up the original harddrive with XP already on it, and now it wants to activate again?
If the second option is what you did, then yes it does need to be activated again, so hook it up to a modem line and it will automatically dial the 800 number to MS and activate itself again.
If XP has been installed on that harddrive longer than three or six months, it will go through the activation without any problems.....however, if it's less than that old, you will be given another screen that has another 800 number for you to call so the techs can give you the new activation number. They will stay on the phone with you while you type it in and get it right and get the thumbs up sign before they hang up.
This is normal activity for XP because any major upgrade in hardware will cause it to need activation again.
I had to replace a motherboard and power supply on my friends compaq presario and since but you have to reinstall Windows XP( it says its already activated ) and brings you user login and when you click sign in, it asks to register XP again(probably windows way of making sure everyone can use the same disk).... so i tried putting a Windows XP backup disk so i can reinstall but it wont bring up the "boot off disk", anyone know of a way of making boot from disk so i can reinstall....
note:Cant log in