ScanMyReg is on GOTD today... Like clockwork, that means that someone(s) will say: "If it aint broke, don't fix it." The definition of that statement is: "I'm happy with what I've already got." Fine... Whalers were happy when people still used whale oil -- stable owners & horse breeders/ranchers were happy when people relied on horses for transportation --- and I'm certain the advancements over the last several hundred years would not have happened if just everyone had stayed pleasantly satisfied. And as I said, if you're satisfied, that's fine -- just don't tell everyone else they should be, 'cause then you'd likely deserve the [not all kind] thoughts headed your way from all those who disagree with your satisfied point of view.
[If my stance on that seems a bit strong, please consider that this illness that's disabled me for the past 20+ years might have at least been treated if not cured, if it were not for the satisfaction with the status quo on the part of healthcare providers & the US gov, which for a number of years not only forbade gov research, but actively discouraged research in other sectors.]
That explanation of sorts out of the way, ScanMyReg is more of a traditional Chinese-type app, meaning non-resizable window, few if any registry entries, a minimal effect on Windows, and more than usual backwards compatibility because of all the XP machines in Asia. In this case you get a few minor files added to C:\Windows\ System32\ [e.g. unicows.dll, w95inf32.dll etc.], & the program's main files in C:\Program Files\ ScanMyReg\ or C:\Program Files (x86)\ ScanMyReg\ , together with optional Start Menu &/or desktop shortcuts and a registry key for uninstall.
The ScanMyReg backup is to a single file, so pretty much of no use if Windows won't run. Defarg [mentioned later] seems to work. A registry scan found 250 some entries in my mostly virgin VM, & they seemed legit, on the order of old cache & path entries [e.g. history] referring to files & folders that were temporary & no longer exist.
ScanMyReg [or similar apps] may or may not help performance on your Windows device -- the odds of it helping increase the longer an install of Windows has been running [as in years], the more it's been updated, & the more software that has been installed. The only way to find out is to try it -- the way to do that with safety is to perform a disk/partition image backup 1st that you can restore [which you should be doing anyway]. Windows 10's registry is different, & ScanMyReg does not list Windows 10 compatibility, so if you try it with 10 I'd suggest that backup even more strongly.
With registry cleaners all you can go by is that many people have said that they help, or have helped them, so if you've got the time & a backup, find out for yourself. After all, since GOTD is saving you the $40 it would cost to find out otherwise, if you've got that time & backup, today might not be a bad day to find out.
As far as all the warnings go, maybe this will help?...
Windows' Registry was originally a central storage place or space to take the place of separate configuration files for every program & Windows itself, not that those .ini files went away completely -- still see them in the Windows folder of 10. BUT... Microsoft doesn't limit what can & can't be entered into the registry, so it's often like the junk drawer in many a kitchen -- whatever anyone wants to put there, they do -- even malware [that way there's no file to trigger AV software]. There are rules, sort of -- more like common sense or practice stuff that's not enforced either.
Windows' Registry can get big -- spread out over several files in different folders it can easily top 300MB. It can take a bit of time & lots of CPU to parse -- to get an idea, open regedit [e.g. type regedit into the run box] & search [in the Edit menu] for a word you make up -- that way you'll search the entire registry rather than stopping when search finds a match. Now it doesn't take as long for Windows or software to read & write specific keys, but you'll get the general idea, that the more there is to the registry the more of a drain it is on your device & Windows. How much of a drain varies along with the registry's size & the amount of hardware resources available, e.g. this is the sort of thing I worry about on my very low powered Windows tablet with very little RAM, much, MUCH more than on my regular PC.
Now you'll read all sorts of warnings [along with some misinformation] when it comes to the registry -- realistically those warnings are very similar to the old: "Don't run with scissors!" Yes, If you change or delete a registry entry that software [including Windows] needs, it may not work properly, or at all, So don't change stuff in the registry randomly.
If you run Regedit you'll see the registry is divided into 5 major keys, each with more [sub] keys, with each of those keys having more [sub] keys & so on -- the hierarchical structure is a lot like the folders, sub-folders, & files you see in Windows Explorer. Each key is like a folder, & instead of files in folders you have individual registry values. You can right click on most any key & export it, which is how you back a key up to a .reg file that can be "merged" [added] into the registry, usually just by double clicking it. NOTE: exporting keys works Much better when the key you're exporting itself contains fewer keys & levels of keys.
ERUNT is an old tool that lets you backup the entire registry, & it lets you restore that backup in Windows, running another copy or installation of Windows, & after booting to a Command Prompt. Setting a Windows Restore Point backs up the registry, but restoring a Restore Point doesn't work 100% of the time. With either ERUNT or System Restore you can have problems if something like your AV software updates afterwards, so always update that, then do one or both of those, & that may help. The only foolproof method however is a disk/partition image backup that you can restore.
Now registry defrag is essentially re-writing the existing registry entries to fresh files getting rid of accumulated white space -- ScanMyReg defrags the registry, & ERUNT includes a tool called NTREGOPT that does that. Depending on how long that copy of the registry has been used, how many updates were performed, & how much software installed & removed, it should shrink the overall size of the registry on disk, though generally not by all that much, making things a little faster, but probably not enough that you'd notice. Registry defrag *might* eliminate some registry file errors [if there are any] by writing new files.
Registry cleaning is a combination of searching for common errors & problem keys/values that are *usually* pretty safe to remove, but beyond that sort of hard-coded list, cleaners can only look for the registry equivalent of bad shortcuts... If an entry in the registry points to a file that does not exist, or another registry key that does not exist, it *should* be perfectly safe to remove that entry. One reason for the long-winded bit about the registry is to point out that without any enforced rules, software can use registry entries that do reference nonexistent files & registry entries. There's no *good* reason for them to do that, but it happens, & when a registry cleaning app deletes that sort of entry, it *may* break that software. OTOH *sometimes* the software it breaks was causing problems, e.g. it was something leftover from an app that was removed, & the regcleaner is said to have performed a miracle cure. It happens.