file gets rename

Hi,
in my office one of the user has windows 98 system,its giving rundll32.exe error as usaual i copied that file from another pc it could work for 2 to 3 min then it gives same error so i cleaned virus, ran sfc & i even reinstalled os.
But no use because after 2-3 min it is renaming rundll32.exe to run32.exe.
please give any solution.
thanks in advance