Corruption

How does Windows "know" if a file is corrupt? John

When files are stored there is a checksum (an extra sequence of numbers) stored with it. When it is read back if the count of zeros and ones does not properly get offset by the checksum then it knows somewhere in that file some bits have been lost/switched.

Is there a copy of each file somewhere where WIndows 7 compares it to? So that it "knows" that a file is corrupt? Where does it store those files? John

To be honest, I don't know exactly; never needed to know, as long as it does what it should.

Why are you asking? Are you getting an error? You can run sfc /scannow to try and repair them or you can put in the W7 DVD and run a system repair.

They'll usually be stored in the C:\Windows\winsxs folder, as far as I'm aware.

They are some more here:
C:\Windows\System32\DriverStore\FileRepository
But they are limited to known OS files.
Tm's explanation is, however, correct, should you get the message that the chksum is in error on any files.