XP uses a complicated formula to determine if it's on the same PC. A change in hard drive alone, IF you have not made many other changes to the system, should not nullify your activation. If you've changed other hardware in the computer, then it may deactivate it.
When you copy the drive you need a program that can copy filesystems in a very specific manner. Many of the major hard drive manufacturers include them with retail drives (OEM may lack them, but you can probably download it, not sure). With earlier versions of windows, like 2000, you can transfer when you upgrade your hard drive without any issues using these programs.
Unfortunately, Windows is very picky about where on the drive certain files are found. Some os's, like Linux, allow you to simply format the partition and copy everything over with a simple copy command. This will NOT work with Windows of any flavor, or even DOS, as the locations of some key system files will be wrong.