You copied the files, but didn't copy the master boot record or the partition boot record, which are necessary parts of the computer's startup process. These are not files, they're raw data structures and assembly language code on raw sectors on the disk. You can't copy these using file copy methods.
What you're doing is, you realize, a totally unsupported way of installing an OS?
As long as you realize that what you're doing can't be supported and is Not a Very Good Idea, you can attempt to use the FIXMBR and FIXBOOT commands from the recovery console to write the MBR and the partition boot record. As reicherb says, you need to also ensure that you've marked the appropriate partition active in the partition table in the MBR.
These low level details are all taken care of properly when you install XP using the setup program. And is one of the reasons what you're doing is not a very good way to move an OS from one drive to another.
If your goal is to move the OS from one drive to another, then use ntbackup to back up all your files (to another local drive or a network drive), swap drives, install XP, then restore using ntbackup. That's probably the safest way to accomplish this.
