What are Windows registry? Why are they so important for your operating system? What can you do to keep them reliable?
According to Microsoft site, registry are "a central hierarchical database used in Microsoft Windows ... to store information necessary to configure the system for one or more users, applications and hardware devices." You can add to that data regarding file types like what application is used to open them, what icons should be display for them and so on.
I will try to explain what's happening on 2 of the most common scenarios that take place on your computer:
1. you get a new program (software or game). You want to give it a try, so you install it. At that point, new data is written in your windows registry. That data include program folder, associated files, various settings that are used by the program, whether it will run on startup (HKCU/Software/Microsoft/Windows/Current Version/Run/) or only at the first startup (HKCU/Software/Microsoft/Windows/Current Version/Run Once/).
No problem so far. But what's happening if you decide to uninstall it? That's that point where things get tricky.
Even if you receive a "successfully uninstall" message, pieces of data remain in your registry. Of course, this is not happening on all cases, but sadly it does in many of them.
In a worst case scenario, if for instance files "*.abc" are registered to an application that you just removed and the whole registration process wasn't properly removed from registry, you will get an error when you try to run that sort of files. Those invalid registry entries won't bother you visibly all the time, but they will cause your computer to slow down or even crash in some cases.
2. You install some new hardware, like another network card or something. When you plug it in, Windows will detect it and install the best drivers for it. Of course, a significant quantity of data is written into registry. Again no problem so far. When you shutdown you computer and remove the just installed device, registry entries are not deleted. They just remain there. If you try, after a period of time, to install a similar device, conflicts may appear, because you computer might confuse them. This is happening because of the old and possibly corrupt registry entries.
A solution to avoid those kinds of problems is to create regular backups of your entire Windows registry. That way you can easily restore them the moment you feel something is not working right. But be careful, because an old backup might cause your newest programs or hardware devices to malfunction. Another way to solve the problems is to use a registry tool that would parse your registry and fix all invalid, missing or corrupt entries that it could find.
You can visit Core Download, a software archive with more than 23.000 programs where you will find a variety of registry tools that can fix all your problems.