The encryption key used for copy-protected content is time-based. As time moves on, the running currently installed Windows 7 system can decrypt (and play) all copy-protected programs recorded previously.
When you take a system image, the decryption capability implicit in that system image is limited to being able to play back all copy-protected programs recorded prior to the date/time of that system image.
So if you restore that system image, it will only be able to decrypt and play back copy-protected programs which were made prior to the date/time of the system image you just restored. Any copy-protected programs recorded AFTER the date/time of the system image you just restored will NOT be able to be decrypted and played back. That's just something we have to live with, so it's often necessary to "view down" any unwatched copy-protected recordings made after the date/time of the system image you're about to restore, because once you do that restore you will unfortunately not be able to view them.
This is the nature of the time-based encryption method and DRM technology built into Windows 7. Very tight, and strict. And there's nothing you can to do get around it.
Same with what will happen if you install a new Windows 7 system from scratch, even on the current identical machine hardware. Once again, you will NOT be able to play any copy-protected programs recorded using the prior installed Windows 7. Once again, the encryption mechanism is not only time-based but also is a function of the hardware and the "internal identity" of the installed Windows 7 system. Installing a new Windows 7 creates a new "internal identity", which is the same reason you can't play a copy-protected program recorded on one system using Windows 7 on another machine. Different "internal identity" makes it impossible to decrypt.
Very tight, and very strict... when it comes to copy-protected content recorded from a cable company service through cablecard-enabled TV tuners and WMC. There is none of this with copy-freely content (e.g. OTA/ATSC programs).