If you have the SP1
slipstreamed CD, then Run: sfc /scannow
(You could use the same procedure as below for the non-slipstreamed CD, but I suggest this just to be sure any supporting files, like Help, are also replaced.)
If you don't have that CD, but have the original XP one, then try extracting the defrag program from the original CD.
If "X" is your hard drive letter, and "Y" is your CD drive letter, and "Windows" is your installation directory (substitute "Winnt" if you upgraded and that is your directory), then Run:
expand Y\i386\defrag.ex_ X\Windows\System32\defrag.exe
The normal XP defragmenter is a version of Diskeeper, so installing Diskeeper replaces the original defrag program.