Having turned on my Windows 7 Ultimate x64 I found my user accounts to be missing and was left with an Administrator account. It seems that the system has reverted to a very old restore point. I have tried using the restore points (recent ones are displayed, but I get the 0x80070057 error). My data seems to be intact.
I have Windows and all programs stored on an internal SSD-drive and the data on an internal harddrive. I also have a recent Acronis image on a NAS. I am hoping there might be a way to solve the problem without using the image.
I have tried booting in Safe Mode, but the restore points are still not functioning.
EDIT: I remembered that I had used a new USB-stick which would not eject correctly, before turning off the computer. I tried reinserting the USB-stick before restarting the system and... everything went back to normal.
This is great, but even though I am now able to eject the USB-stick correctly, the problem comes back when I restart without the USB-stick.