Windows XP was released in 2002, making it a very dated system. This means that if you want to find the best registry cleaner for it, you need to be able to use the tool that is still able to clean out the XP registry quickly and reliably, whilst not causing any more damaged.
Unfortunately, many registry software are now so advanced that they cause more damage than good to XP. This is because all of these tools work in a similar way to clean out the 'registry' database of Windows. This is a central database which stores information about your system and its programs. The reason why many registry tools will cause more damage than good is because this database has changed dramatically over the past few years, and using an incompatible registry cleaner on XP will just make it try and delete a lot of important settings that your computer needs.
The best XP registry cleaner is the tool that can scan through the XP registry database and remove all the damaged and corrupted files that are inside there, without causing any more damage. These tools work by scanning through every registry file inside the database and fixing any of the ones that they see as corrupted. Corrupt registry files cause Windows to run slower and with a lot of errors, as they become unreadable. Windows has been designed to use registry settings & files every time you use your system... so when it cannot read any of them, it has to slow down whilst it processes the files it needs.
We've been using a lot of registry cleaners and have found that there are a lot of tools which no longer work on XP any more. We've found that only a handful for cleaners are designed well enough to still work well on XP, and the best one is a tool called "Registry Recycler". It is extremely popular thanks to it being able to find and fix the largest number of registry errors on your system. It's also able to work extremely well with XP, allowing it to quickly fix the largest number of problems on this system.