Windows XP is smart enough to analyze user behavior and adjust accordingly (somewhat !) Important
example is MRU list. Periodically (default is 3 days I think), XP would do some file movements to
keep most actively used programs in that part of the disk that are quickly accessible. (Believe me all
parts of disk are not the same,some are quicker to access and some are not..its simple law of physics).
Anyway if you want to do it at your will, go to START-RUN and enter Rundll32.exe
advapi32.dll,ProcessIdleTasks and start the reordering process.