Ashampoo's registry cleaner is on GOTD today. There aren't many comments on what's usually a controversial topic, moderation being light on weekends, but with the one sane post being down-voted, I figured spending a few moments talking about Windows registry, registry problems, registry cleaners etc. might be of use. Bottom line [if you want to skip all the details]: the longer it's been since a copy of Windows was installed, the greater the odds a registry cleaner *might* help -- if you perform a disk/partition image backup beforehand that you can restore, running a registry cleaner is pretty much risk free. Nothing ventured, nothing gained.
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An Operating System [OS] -- the software that makes PCs & cell phones etc. useful, allowing them to run software programs or apps -- needs a way to store settings, for the OS itself, for the software that runs on the OS, and for the user, spelling out what they're allowed to do, and not, along with their preferences, things like phone numbers and so on. Linux-based OSes store this information in [usually text-based] files -- Windows stores this data in a combination of files and the Windows registry, which is basically a database spread out over several files.
The registry, being more complicated, is more trouble prone that data simply stored in configuration files. And Windows registry is also [obviously] part of Windows -- to take the view that Windows, or its registry, are designed to be trouble-free, and therefore can never have or be a problem, is unrealistic to say the least. We're reminded of that 2 or 3 times a month when Microsoft pushes out patches via Windows Update. Or put another way, how can anyone assert that Windows registry cannot be a source of problems, when the OS that the registry is part of is so imperfect?
Windows registry is large, and there are no rules enforced on what can be entered into the registry, or where stuff can be entered in the registry, which is divided up into several sections -- one large section of the registry isn't even displayed in the default app for registry editing, Windows own Regedit. Windows 10 registry is larger & more complex than the registry in Windows 7, which is larger and more complex than the registry in XP. The registry is read when Windows starts, and then portions of it are read &/or written to constantly while Windows is running. Size and degree of fragmentation then have to matter, but whether any delay is actually noticeable is another story -- sometimes yes, sometimes no.
The only fully comprehensive way to remove those entries that were added to the registry when installing a program, is to work from a list of changes recorded using an app like Regshot, manually deleting those entries in Regedit. One Gotcha is that Regshot doesn't work reliably in Windows 10 -- another is that many of those new registry entries may be garbage, like temporary logs or cache. There is no listing or source to tell you every sort of entry that's trash, which ones matter, and which ones are risky to alter or remove. Regedit does have a search function, but simply searching on an app's name will often not bring up every entry its installation added to the registry. Often installing software will add far more registry entries that you could delete individually in one or two days.
When you start Regedit [type regedit in the Run box], you'll see the Windows registry displayed in a multi-layered outline format, with "Keys", and "Values" listed under keys. If you search for an app's name, you may find it in a value -- often the entire key is only related to that app and can be deleted, but not always. You have to make that judgment call. An uninstall app or registry cleaner might perform the same sort of search for you, but will only delete the value, leaving the rest of the key behind. Otherwise these cleaners will look for broken links, like registry entries listing a file name and the location of that file -- if the file isn't at that location on the hard drive, that entry will be flagged for deletion.
Often part of the process is to re-write the registry, which reduces fragmentation. Running a registry cleaner therefor may help by reducing the size of the registry, by reducing its fragmentation, &/or by removing a problem entry, even if accidentally, e.g. it may remove an improper entry that's required for a misbehaving program to run. By the same token, it may break a program that you want, if/when that app depends on a registry entry with a broken link, and yes, it happens... let's say v.3 of an app required a specific file, and included checking the registry for the link to that file -- v.4 of the same app no longer uses that file, but since v.4 is built from v.3, the code creating that registry entry & the code checking for that registry entry still exist.
Microsoft's answer to the problem of overgrown, problematic registries, has traditionally been to regularly re-install a fresh copy of Windows -- with Windows 10 it's less of a problem since Windows gets re-installed twice per year. When you upgrade to a new version of Windows 10, the setup process tries to keep all the registry entries you need, and tries to delete the ones you don't. It's not always successful, but generally does a good enough job.
Running a registry cleaner may or may not help -- the longer it's been since that copy of Windows was installed, the greater the odds it'll help. The only way to eliminate all risk however is to have a complete image backup that you can restore.
Some of the comments today recommended apps from a tweaking site... The only tweaking-related apps you might want to use in Windows 7-10 just give you a unified GUI for stuff like settings that are already native in Windows, but spread out across several apps or settings otherwise. You can certainly add stuff, like a Windows 7 style Start Menu to Windows 10, but you'd be very hard pressed to make a fresh install of Windows 7-10 run better/faster -- the tweaking apps that were useful in Windows 95 just aren't anymore. You can also skip apps promising to remove or erase your tracks for increased privacy, though you *might* find apps useful that make it easier to set Windows 10's privacy settings... use the privacy mode in all major web browsers to prevent saving tracks in the 1st place -- accept that the only sure way to avoid a history of what you've run in Windows is to use virtualization software, e.g. Shadow Defender, or use a VM, destroying the *copy* of the VHD you used containing the VM afterward.
Personally I find any software with the word "Repair" a bit scary. A software developer could only target known problems, because those are pretty much the only ones anyone hears about -- that developer would have no way to know if or how or why your PC was misbehaving. And if it's a known problem, there's info published online, and normally soon enough, a fix. When I have a problem I Google, usually find info, either find a fix or find I have to wait for one, and apply it when available. I just don't see where an app would/could help in that scenario. So-called repair software might look for common adware or malware, or maybe invent problems I didn't know I had, but I just can't imagine any universal, automated fixes that I'd want applied to the systems I'm responsible for.
Please remember that a bit of skepticism is useful online... [While it seems to me an effort in the comments to promote a site & its software, still No Offense Intended] When someone says [in the comments], based on my experience over many years, they may have loads of experience running a large IT shop, or maybe they've browsed the web a few times over the last 5 years. Someone saying they're a retired IT tech tells me nothing -- they could have done network maintenance in a shop running all Linux servers, or he may have retired when win95 was current. A web site [or something like a YouTube video] with useless info, &/or a site offering useless software, even when free, can still make its owner money because of ad revenue. A web site might monetize user data, harvest email addresses, or in more extreme cases, use your CPU for mining.