I recently came across somebody posting on the Microsoft support groups who had problems with one of his input devices misbehaving. It seems one of the things this person tried to do to resolve this issue was to purchase a registry cleaning application. I won't name the specific one here.
The registry is a part of your computer where Windows and other applications typically store their settings. It is essentially a database, and it is critical to the functioning of your computer.
What these so called "registry cleaners" claim to do is to fix "errors" in the registry. One example this particular piece of software uses is "software you regularly instal (their typo not mine) on your machine is rarely accompanied by uninstall utility and, even when it does, fairly often it leaves broken Windows Registry keys behind".
Well other than the fact that most software does have an uninstaller, the language is overly alarmist. A lot of application uninstallers will leave orphaned (we don't call them broken) entries in the registry. However this is no cause for concern, those registry keys don't do anything, and due to the database like nature of the registry cause no performance impact.
They also claim "This software program will inform you on where the registry errors are located and it will eventually: Eliminate 100% of your PC Errors".
When you hear something like that your scumbag alert should sound. If I had a power surge and my PSU blew, how is this software going to repair that? If I've got blown capacitors on my motherboard how is this software going to fix that? It can't.
This sort of nonsense is so typical of the registry cleaner hard sell. Not only that I can count six download links to the software on their front page. Yes six, they really want this software on your machine. What they don't tell you is the price of this software, they'll wait for you to install it let it do a free scan, then they'll come up with a bunch of so-called errors you have and offer to fix them for a price.
There are registry cleaners from more legitimate sources, such as Windows Live OneCare, who have a free online scanner, which will also fix issues it finds for free too. However do you really want to put your computer at risk? If one of these scanners failed in some way they could take down your system.
My advice is, unless you have a specific problem that can be traced to the registry, and you have a specific address for a key in the registry, and specific instructions on what to do with the key. Leave the registry alone. Certainly never buy software that claims to "clean" it.