How do I delete a protected registry entry?

There's a registry entry that I really need to delete,
I go into Regedit to delete it but when I do, I get a dialogue box saying:

Cannot delete [such and such]: Error while deleting key.

It's not a system entry, just a corrupt AutoCAD installation I can't reinstall or remove.
I've also backed up the registry, so don't worry.
Please help me find away around this, I really need to delete these entries, and a registry cleaner doesn't work.

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