thomas-k wroteI have used "Clean and fix my computer" - aggressive and fix all found problems. After this I wanted to test
"TuneUp Registry Cleaner" and see his results. I wonder that "TuneUp Registry Cleaner" found so much problems.
Different registry cleaners find different number of errors because they use different criteria on deciding what is counted as an "error". While the actual analysis of what is an erroneous piece of registry data is very objective (as in there is very little room for interpretation), it is much more subjective as to what - if anything - should be done about the error.
For example, let's say user has a software called Qwerty by Acme Soft installed to his system. Everything indicates the software is working perfectly. However, it has created a registry entry that is erroneous. Since the software is installed and working fine, should we do something about the erroneous registry entry? The design philosophy of jv16 PowerTools has always been that even with its most aggressive setting, safety comes first. Basically, jv16 PowerTools does not list anything as errors unless it has an extremely sound case for believing that fixing that error would be beneficial for the user.
Another reason is that there are a lot of duplicate data in the registry (for example, the entire HKEY_CLASSES_ROOT is simply a mirrored duplicate of HKEY_LOCAL_MACHINE\Software\Classes, meaning any data under HKEY_CLASSES_ROOT is also in HKEY_LOCAL_MACHINE\Software\Classes and vice versa) and registry cleaners can give an impression of more errors being found by listing each of these duplicate data individually. Listing these items individually makes no difference in the results of the registry cleaning or the operation of your computer, but it allows the software to produce a longer and more impressive looking list of results.
And thirdly, "scareware" products also create fake data in an attempt to scare the sure about the bad state of his computer in an effort to try to scare him into buying the product to clean or fix these imaginary problems, thus the name "scareware". Though this is a general comment on the subject, TuneUp Utilities, for example, is a well respected product and wouldn't do such a thing.