The Microsoft Windows registry is a hierarchical database of system configuration data. Many of the programs written for 32-bit and 64-bit Windows platforms require registry data. This means that it is used frequently and that some programs really like to tamper with the registry. Running a program like RegMon from SysInternal, which logs access to the registry, can show you how much the registry is being used by some programs. Even Explorer, the built-in file explorer in Windows, is heavily used and this can have a huge impact on performance.
When Windows is first installed, the registry is relatively small. It should not contain any extraneous data and the data file should not be bloated or highly fragmented. However, over time this can change, especially when many programs are installed and uninstalled. When programs and other components are removed from the system, they can leave behind data within the registry.
Many programs have been created to erase data left behind by improperly uninstalled programs. The original, RegClean, was developed by Microsoft. A web search will bring up many results for registry cleaners, some sites touting their commercial solution as a panacea for any Windows problem. Choosing a registry cleaner is best done with good advice, and the forums are a great place to search and ask what cleaners people are using. I suggest you look for a shareware or freeware cleaner, it’s not worth paying for one when there are excellent freeware ones available. RegClean is available from the Microsoft Download Center [http://www.microsoft.com/downloads/search.asp].
Once data is removed from the registry, the space in the file used by that data is kept until it can be reused by newly added data. If many very small pieces of data are deleted, the registry can become very bloated with “holes” where no data will fit. This makes the registry larger than necessary, which in turn means that it takes up more space on your hard drive and is therefore slower to access. To solve this problem, there are log compactors that can rewrite new log hives without holes. I use ExperimentalScene RegCompact.NET to compact my registry, with great success. My system noticeably speeds up when the registry has been heavily bloated and I compact it.
Once the registry has been cleaned up and compacted, it should also benefit from optimization. This is done by rearranging the location of the registry hives as they are stored on your hard drive. Like normal file optimization or defragmentation, this process will further improve registry speed.