cannot format windows..

-the pc is ok(can run all programs in the windows)
-as many users use this pc,it has some problem about registry/system file and so on.
-so i try to install the fresh copy of windows xp
-i put the cd and boot from it.
the system loads files as usual
-once i press Enter to format(or R),then the system shows some sentences which mean like:
.....cannot setup windows,the disk is not found,press F3 to quit...
-then i go to windows again.i found that some files such that 'ntldr','ntdetect.com' placed in local d: (not in local c: ).i cut that file and place it in local c:..
-now,it shows ntldr is missing(i know how to fix this).the major problem is,why it cannot find hard disk?and how to fix this?