1.) What do such programs determine to be "errors?"
2.) How do such programs "fix" such registry "errors?"
The answer to both is that they scan the registry and attempt to find registry entries which are unused, then delete them. Most of those "errors" are actually references to files extensions, applications, et cetera which are not frequently used, but are still used nevertheless. Thus, you tell the cleaner to delete the entries, Windows recreates them days later because it needs them, and then you delete them again. Or worse, you tell the cleaner to delete the entries it detects are "errors" when the entries were actually required for an installed program, or even Windows, to run. In that case, you've crippled an application in order to save a few bytes of space and not increase system performance. If you ever wonder why 'perfectly tuned' computers spring leaks, those tuning utilities are often to blame.
In short, registry cleaners used to have some significance 'back in the day,' but on modern systems they cause more harm than good. Some of the developers of registry cleaners are even admitting that fact concerning Windows Vista and Windows 7.
Ever sense my infancy stage in first using and understanding the " great " computer, and my first model was a Commendore which was only view with DOS,It seemed that no matter what safe guards and security and performance enhancements you applied,each and every time you engaged a program or feature is the exact moment that your stability was dying a slow death by way of a deteriorating registry. We are now advanced in our technology and still out of all the programs and software's available, including third party versions, there are 10 times more programs and software's, which address the system registry to help maintain a stable environment. Example., you can install even a minimum of two (2) Registry help software's and run their scan features and 9 out 10 times, your first scan results will show approximately over 2000 registry errors and missing or invalid keys. Now you ask yourself, I just purchased this brand new computer with dual core processor and state of the art electronics and I have only turned it on for not even a full days use, and I have this many registry errors...So you say, well, first I am a novice computer user and I am just going to take for granted that something is not right, so let this program cure these ill's. I now began to be a little curious. I am somewhat of an explorer and want a second opinion, so i click on the second registry fixer program, knowing with my own eye's that the first one took about 30 seconds to " Fix " these errors and If I run this program right after the first scan and repair, I should see absolutely " Zero " errors, ' Right ', I run the second scan, and to my awe, there are roughly 294 errors. Now, I grant you one sign of relief, Windows 7 out of all the operating systems I have come to love and hate, registers the least registry errors with in a weeks use, and I intentionally keep my computer on with the screen saver set to engage after 30 minutes of inactivity. I use Windows Advance System Care Pro and set the feature to automatically scan and repair the registry everyday a 5 PM and I use the Smart Fragmentary program to run in the background continuously. This method greatly improved the performance of Vista and Works Great with Windows 7, For I fear if I did not provide some type of assistance to help with the performance issue that arrive, My experience with Windows 7 would be just as frustrating as Vista. Now again, Mind you, I love the vast improvements of Windows 7 and I have been a faithful Beta user to date, But I seriously ask, When will the geniuses at Microsoft and all the other Mac and Linus, etc, etc began to address the Issue of a more Stable registry, which is the lifeline of any well tuned operating system, or is the problems associated with a Registry like that of us drinking a glass of water, knowing that we will need another glass soon or later, or else we " Die ". Long live Bill Gates and his effortless Dreams.