The Windows Registry - A Resource Guide
A number of articles in the FAQ reference the Windows registry. For those interested in more detail, here are some references that might help you understand what the registry is and how to work with it.
Please note that any changes you make to the registry, if made in the wrong way or wrong place, have the potential to make your system unstable and/or unbootable. The changes suggested on this site have been tested with the versions of Windows they refer to and should not have that effect; but anything is possible with computers as they are not appliances and are sensitive to how they are set up and what software you have installed on them. With the amount of software out there it would be impossible to test all combinations for unintended interactions.
Always keep a good backup you can recover from and keep it current.
Microsoft Knowledgebase Articles
That should get you started.
If you have Windows Vista, know that you have more backup options. They are described in this article...