You're talking about taking an NTFS formatted drive and physically moving it from one XP system to another? Of course you can do that without issues. Just make sure you setup the drives right, jumpers, cables, etc. It should boot in the new system like normal, except XP may "install new devices" and want to reboot right away (or it may not). It will assign the drive the first available drive letter, which you can change later in the MMC.