/p: Normally, a disk that doesn’t indicate problems will not be checked exhaustively. Use /p to override this indication and run the exhaustive check anyway. CHKDSK does not make any changes to the drive when run only with /p.
/r: Locates bad sectors and recovers any readable data. /r automatically includes /p.
CHKDSK requires the file Autochk.exe in order to work. If it cannot find it in the system folder (usually \Winnt\System32 or \Windows\System32), it will look for it on the Windows Installation CD.