Hello Peter and welcome to Seven Forums.
From the ERUNT FAQ section:
Q: Do ERUNT and NTREGOPT run on Windows 7?
A: ERUNT and NTREGOPT in their current versions 1.1j are still compatible with Windows 7, but as in Vista, they will only work correctly if you turn off User Account Control in Windows' Control Panel (move the slider to the lowest position). Also, a problem has been discovered which on many systems causes ERUNT and NTREGOPT to display a "RegSaveKey: 3" error when optimizing / restoring the BCD00000000 hive. The cause is that after a clean install of Windows 7, the BCD part of the registry which contains Windows' boot configuration data resides on a hidden system partition with no drive letter assigned in Explorer. You can simply ignore this error and continue, or as a workaround, open Disk Managemant in Control Panel and right-click on the partition displayed as "System Reserved" to assign a drive letter. ERUNT and NTREGOPT FAQ
ERUNT is a registry backup and restore tool with potential problems in Windows 7. You can use the built in Windows 7 system restore to create a registry back up. Registry - Backup and Restore
Also, since you're already doing system images with Acronis, I think I'd forget about using ERUNT unless there's some kind of overwhelmng reason why you have
to use it. If something corrupted the registry and you feel the need to restore the registry, I think the chances are pretty good that other programs and/or files may also be corrupted.