Software support

I have a PC running on Win XP,it suddenly stopped booting reporting that Windows\system32\config\system file is corrupt or missing.I then manually replaced the files SYSTEM and SYSTEM.SAV to the folder from one one hard drive onto the problem HD .When I tried restarting the system it gave me the deadly Blue screen,on trying to run chkdsk /f the computer asks for the Administrator password but rejects the blank password which it had been accepting before.Could it be files copied which changed the password.What could be the best solution?