Windows 7: System File Checker (sfc), Options?

System File Checker (sfc) has a few parameters that you can use when runing sfc:
Parameters
/scannow : Scans all protected system files immediately.
/scanonce : Scans all protected system files once.
/scanboot : Scans all protected system files every time the computer is restarted.
/revert : Returns the scan to its default operation.
/purgecache : Purges the Windows File Protection file cache and scans all protected system files immediately.
/cachesize=x : Sets the size, in MB, of the Windows File Protection file cache.
/? : Displays help at the command prompt.



What's the difference between the first two, /scannow and /scanonce?
Does /scanonce wait until you reboot and then run one time?
And while I'm asking about sfc, where is the "Windows File Protection file cache"?

/scannow scans all protected system files immediately. /scanonce scans all protected system files at the next boot.

System File Checker

How to use the System File Checker tool to troubleshoot missing or corrupted system files on Windows Vista or on Windows 7

You might find this Microsoft KB article interesting re: Windows File Protection.

Description of the Windows File Protection feature

And option three, this tutorial, gives some good info on how to see and read the CBS.log if a System File Checker scan finds any problems.

SFC /SCANNOW Command - System File Checker