The Windows Cleaning Registry is "The configuration database in all 32-bit versions of Windows that contains settings for the hardware and software in the PC it is installed in," according to the TechEncyclopedia. That's actually a pretty good thumbnail description. Wikipedia has a somewhat more detailed definition, which starts this way:

In computing, the Windows Cleaning Registry is a database which stores settings and options for the operating system for Microsoft (NSDQ: MSFT) Windows 32-bit versions. It contains information and settings for all the hardware, software, users, and preferences of the PC. Whenever a user makes changes to "Control Panel" settings, or file associations, system policies, or installed software, the changes are reflected and stored in the registry....

In all versions of Windows, the Cleaning Registry can accumulate obsolete and nonfunctioning entries. Older versions of Windows, in particular, were notorious for this, often ending up with Registries containing huge numbers of erroneous entries. This slowed computer operations; and could even lead to crashes.

This led to the creation of a number of third-party software utilities that purported to "clean" the Registry, finding and eliminating the extraneous, useless, or just plain bogus data stored there.

Even Microsoft eventually got into the act, producing a small, unofficial, unsupported tool called Regclean for the Win9x family of operating systems. Microsoft made several half-hearted attempts at improving Regclean over several years, but Windows ME was then in the works, and it's Cleaning Registry was different enough from the earlier Win9x family to require that Cleaning Registry tools work differently; and Windows NT/2000/XP required wholly different software. Because of this, Microsoft quietly abandoned Regclean, never having officially released it as a supported product. (It's still around on various non-Microsoft Web sites, however.)

Today, third-party Cleaning Registry tools abound, filling the gap that Microsoft left. And that's a good thing, because even though XP is much harder to crash than was Windows 9x, it still can benefit from some Cleaning Registry. For example, when you upgrade a PC from Win98 to XP, the new operating system will carry along a boatload of Cleaning Registry settings from the old setup, just in case they might be needed. While this helps ensure that the new setup will work, it also virtually guarantees that the new setup carries excess baggage from Day One.

A brief aside to prove the point: On a recent "House Call" (an on-site PC-diagnostic and repair session at a reader's place of business or home office), one reader had upgraded a major-brand PC from Win98 to XP. This reader had above-average PC skills, and had regularly backed up his system, defragged, and otherwise maintained it, including running Norton's "WinDoctor" and ToniArt's "EasyCleaner" to help maintain the Cleaning Registry. Despite all that, we found and were able to remove some 3,000 additional bogus entries in his Cleaning Registry. That, and some other maintenance steps we took, cut his PC's boot times in half, and made the system perceptibly faster and more stable. The full House Call repair is a separate story we'll tell at another time, but the salient point today is that, even using two Cleaning Registry tools, he still had some 3,000 needless entries bloating his Registry and bogging down his system.

Nonupgraded systems also can accumulate some unnecessary Cleaning Registry bloat, too, especially as hardware is added or removed, and software is installed, uninstalled, and upgraded. The more dynamic your system use, the more likely that your Cleaning Registry will eventually accumulate a nontrivial number of obsolete or otherwise bogus entries.