Hi loustalou1:
This won't help you now, but for future reference there are instructions for restoring registry backups created by CCleaner at How Can I Restore Registry Backups?.
The following warning is posted in the CCleaner support article Clean the Windows Registry:
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"<em><strong>We recommend examining Registry Cleaner's fixing options carefully</strong>.</em>
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<em>This is important if you select parts of the Registry that you want to clean (see screenshot) but are unsure what will happen if you remove them. We've made CCleaner's fixing options (listed beneath Registry Cleaner) in Registry > Registry Cleaner to help reduce potentially harmful results. <strong>It’s always safer to leave an entry than to remove it if you’re not 100% sure.</strong></em>"
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There have been several posts in this CCleaner forum where the CCleaner registry cleaner damaged software, so as a general rule I only use this registry cleaner if I have uninstalled a program and need to remove orphaned files or registry entries that are causing a problem (e.g., interfering with re-installation of the program or causing a conflict with installation of another program). The Microsoft support article <em><a href="https://support.microsoft.com/en-us/topic/microsoft-support-policy-for-the-use-of-registry-cleaning-utilities-0485f4df-9520-3691-2461-7b0fd54e8b3a" rel="external nofollow">Microsoft Support Policy for the Use of Registry Cleaning Utilities</a></em> also warns that:
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"... <em>Some products such as registry cleaning utilities suggest that the registry needs regular maintenance or cleaning. However, <strong>serious issues can occur when you modify the registry incorrectly using these types of utilities</strong>. These issues might require users to reinstall the operating system due to instability</em>..."
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I own a Dell computer, and I normally use <a href="https://www.revouninstaller.com/products/revo-uninstaller-free/" rel="external nofollow">Revo Uninstaller Free</a> in "Aggressive" mode to uninstall Dell utilities like SupportAssist, Dell Update, etc. to try and clean up orphaned files and registry entries often left behind by the uninstaller. Also note that there are dozens of recent threads in the <a href="https://www.dell.com/community/SupportAssist-for-PCs/bd-p/supportassist-pcs" rel="external nofollow">Dell SupportAssist for PCs</a> board about issues updating or re-installing the SupportAssist utility. For many users these problems are related to a major change in the system requirements for the latest <strong>SupportAssist v3.11.4.29</strong> (released 23-Jun-2022) which <strong>now requires the .NET (Core) Runtime v5.0</strong> in addition to the "classic" MS .NET Framework v4.7.2. See my 24-Jun-2022 post in DavG94's <em><a href="https://www.dell.com/community/SupportAssist-for-PCs/Dell-Support-Assist-Fails-to-install/m-p/8223352/highlight/true#M93060" rel="external nofollow">Dell SupportAssist Fails to Install</a></em> for further details.
EDIT:
You also asked "why was I unable to restore the deleted dlls". Note that a .DLL (Dynamic Link Library) is a file that often contains code and data that can be used by more than one program. It is not a registry entry, and the registry cleaner in CCleaner will not actually remove a .DLL file from your hard drive. However, the registry cleaner might remove an important registry key that references (points to) that .DLL file.
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64-bit Win 10 Pro v21H2 build 19044.1766 * Firefox v101.0.1 * Microsoft Defender v4.18.2205.7-1.1.19300.2 * Malwarebytes Premium v4.5.10.200-1.0.1702 * Macrium Reflect Free v8.0.6758 * Dell SupportAssist v3.11.4.29 * Dell Update for Windows Universal v4.5.0 * Inspiron 5583/5584 BIOS v1.18.0 * CCleaner Free Portable v5.91.9537 * Revo Uninstaller Free Portable v2.3.9
Dell Inspiron 15 5584, Intel i5-8265U CPU, 8 GB RAM, 256 GB Toshiba KBG40ZNS256G NVMe SSD, Intel UHD Graphics 620</span>