Ah right I understand where you're coming from now. When the PC boots up, CHKDSK* does a 'dirty' check. It basically checks a tiny flag on the drive. This takes mere milliseconds.
Each time you boot up and run Windows, the OS flags the drive as dirty. When you shutdown cleanly it marks the drive as clean.
Therefore, in theory, if your computer suffers a major crash the system will boot up as 'dirty' and so CHKDSK will do a scan. Needless to say it rarely works...
So everytime you boot up if it's dirty, it does a CHKDSK scan, if not the next program runs in the autorun registy entry. Which in this case is PageDefrag.
Check out the Chkntfs.exe program in Windows XP and above. This allows you to set or clear the Dirty flag.
Every month I run a simply one line batch file via the Scheduler.
ECHO Y|CHKDSK c: /f
This forces a dirty flag on the drive.
If everyone did a dirty check once a month, their computer would work better....
*Technically CHKDSK isn't involved at all, as the Autochk program checks the dirty flag and runs CHKDSK if it is set.