A common misconception in the computer world when concerned with performance issues: lag, a slowly responding PC, etc is cleaning out your Registry with an application will make a huge difference.
The fact of the matter is, they can do more harm than good and render your machine to being used as a doorstop. I don't know about you but I'm rather attached to my computer and I like it functioning .
The key thing to remember is the Windows Registry is where Windows stores everything. Configuration information, how Windows boots, changes to settings, file associations and much more. If this database is compromised in any way the consequences could be disastrous.
More detailed information for advanced users can be found here
Registry cleaners are extremely powerful applications and their potential for harming your OS far outweighs any small potential for improving your computer's performance.
There are a number of them available and some are more safe than others. Keep in mind that no two registry cleaners work entirely the same way.
Each vendor uses different criteria as to what constitutes a "bad" entry.
One cleaner may find entries on your system that will not cause a problem when removed, another may not find the same entries, and still another may want to remove entries required for a program to work.
Without research into what the registry entry selected for deletion is, a registry cleaner can end up being an automated method to cause problems with the registry.
Not all Registry Cleaners create a backup.
The single most important thing you can do when making changes to anything is back up the original copy. Some Registry Cleaners do this, and others don't. If the changes prevent the system from booting up, then there is no backup available to restore it in order to regain functionality.
For routine use by those not familiar with the registry, the benefits to your computer are negligible while the potential risks are great.
Registry cleaning makes malware removal more difficult
When removing malware related entries in the Registry this actually may hamper disinfection efforts. If all outward traces of an infection are wiped away and the infections is improperly removed there will be no further indication of infection.
Overall, the performance increase you will gain by deleting old, defunct and obsolete entries in the Registry is overrated. I would strongly suggest you leave the Registry alone.
Microsoft has valuable information about Registry Cleaners here
Many security experts as well as websites, iHelpForum included, do not condone the use of Registry Cleaners. See more here and here
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Registry Cleaners: Do I really need one?
common misconceptions addressed