I was doing a little amateur investigation and found some interesting points and questions (unless I am misinterpreting something)
1. The userdif file is only used when upgrading to manage user profiles during the upgrade
2. #1 would mean that this is not necessary for running, only for major upgrades
3. The registry seems ok, but the file is corrupt, so is it possible to recreate the file from the registry?
4. What will happen during upgrade if the file is missing? would it continue without, crash, or recreate?
5. When does this file get created? It must not be stock, and if #1 is correct, it would need to be created or at least updated when an upgrade is initiated (to make sure it's up to date) , so can that be forced?
6. this file seems to be a target for malware. I saw quite a few references to malware corrupting this particular file.