since you have different hardware, MS will recognise that and think it is a different computer. this is just to try and prevent piracy.
I believe you have a legitimate XP, so go to activate by phone, call MS, and if the machine says you can't activate, dial the number they tell you if you think you got the message out of error. they should give you an activation code
to format your old hard drive, you can do it many ways. you can go to my computer, right click and click format
you can format through disk management (dskmgmt in run)
you can format through the Windows install disk, or in the recovery console of the install disk