You didn't tell about your old machine. HP and others LOCK the OS to the machine. The OS will never "transfer" and people get a rude few hundred dollar OS shock as they buy a full copy.
Moving your stuff is as good as your backups. Moving a hard disk and hoping the contents stick around is begging to learn why people write so much about backups.
I am planning to build my first computer utilizing a barebones kit from Tiger. I am sure I can do the assembly, the question I have is will I have to reinstall the Windows XP OS on the hard drive after I remove it from the old computer and install it in the new one? I read somewhere that the OS would have to be reinstalled, but I do not want to loose all the stuff on the drive.