Ok, as a good citizen... I used Piriform to defrag C: and ended up with an 80GB loss. Having read the above, I deleted all restore points, did the vssadmin.... but no matter how correct the syntax I only get Error: Invalid option. I used WinDirStat to verify that ALL folders and files are the same exact size as before, so there seems to be no accounting for why Windows Explorer sees less disk space than WinDirStat or SpaceMonger. Maybe the space is still there, maybe not, but it seems bad advice to live with it. By the way, I use a dual boot system with a clone of the Windows 7 OS on drive C: as the alternate boot. I had also run the Piriform defrag on that drive without any change, so none of the above theories add up. I also NEVER use Restore or Windows backup because both are prone to complete failure, Windows backup especially unless you extend the hidden partition from 100MB which you are not warned about. I do images with Macrium Reflect for both boot drives when everything is updated and working fine and thus for seven years have maintained two perfectly working Windows 7 boot drives. About twice a year I get problems on the OS that are very difficult or impossible to fix (usually some kind of virus or this time the Piriform Defrag). Sooo.... I will now go and change the first boot disk in CMOS to the HDD USB memstick that boots the Win PE provided by Macrium, then restore the last image I made two weeks ago, then in the future refrain from using the Piriform defrag tool.