The Registry

You can find the Windows XP and 2000 registry editor by typing "regedit" from the Run command line or opening the Command Prompt window and typing "regedit.exe". What is displayed is all hardware, users, preferences, operating system programs, and PC settings in a tree format. Windows 2000 and XP Pro for instance displays the following Keys under My Computer:

+ HKEY_CLASSES_ROOT
+ HKEY_CURRENT_USER
+ HKEY_LOCAL_MACHINE
+ HKEY_USERS
+ HKEY_CURRENT_CONFIG

These keys contain subkeys, where there is a "+", and yet more subkeys can be nested within these subkeys. The data stored in each key are called values which consists of names and the associated information. Each group is split into Hives which are labeled with "HKEY" which simply stands for Hive Key and a filename indicating what information is stored there. For instance HKEY_LOCAL_MACHINE has the settings for Hardware, SAM, Security, Software, and System. These contain information about the settings for Windows and hardware device drivers to name a few. Some of this data is changed each time the system in booted and is not stored in the files permanently but by each session.
The information stored in the registry files can be edit manually. Unless you are intimately familiar with these files and their contents, it would be advisable to backup each file before editing. These files can easily become corrupted and cause your system to crash. If on the other hand, you venture into editing these files, after backing them up, you can easily delete leftover portions of uninstalled programs that left files behind. This can slow your system down considerably over many months of adding and deleting programs. Some older uninstall programs will leave data in the registry without the use knowing it. Several companies now offer registry cleanup programs and I would recommend visiting a trusted website like Consumer-Review.org obtain a legitimate program to delete unwanted data.