Sometimes you need to check the health of your hard drives by running a diagnostic on them, but you cannot fix any problems because "the system cannot get exclusive access to the drive". This problem occurs more frequently on the drive in which Windows is installed.
So, you schedule CHKDSK to run before boot (or sometimes Windows will schedule it), and it either takes friggin' forever, or tries to run a CHKDSK every time you boot! The problem is that a flag has been set on your drive and isn't being cleared after a successful run. There is a simple procedure to correct this problem, but you have to go through a few screens in Vista or XP to do it.