I find no reason to buy a registry cleaner. There are plenty of free ones that work well enough. In most cases removing hundreds of registry keys will make no difference in performance. Removing one wrong key can leave your system unbootable.
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Registry Cleaners
http://mywebpages.comcast.net/SupportCD/XPMyths.html
Myth - "Registry Cleaners improve performance."
Reality - "A few hundred kilobytes of unused keys and values causes no noticeable performance impact on system operation. Even if the registry was massively bloated there would be little impact on the performance of anything other than exhaustive searches."
Notes - "Registry Cleaners can fix problems associated with traces of applications left behind due to incomplete uninstalls. So it seems that Registry junk is a Windows fact of life and that Registry cleaners will continue to have a place in the anal-sysadmin's tool chest."
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Free Registry Cleaners
Windows Live OneCare
http://onecare.live.com/site/en-us/default.htm
Full scan: Backs up registry and then cleans registry, virus and spyware scan, disk cleanup, disk fragmentation (if needed), updates windows, and checks for open firewall ports.
Clean up scan: Backs up registry and then cleans registry, updates windows.
RegScrubXP
http://fileforum.betanews.com/detail/RegScrubXP/1040020223/1
http://www.majorgeeks.com/download.php?det=2048
RegSupreme Pro 1.4.0.48 (30 day free/ $16.96)
http://fileforum.betanews.com/detail/RegSupreme_Pro/1122042238/1
Finds an amazing amount of entries in aggressive search mode (a thousand plus entries) after using all other cleaners listed here.
CCleaner: Do not install toolbar and recycle bin options
"Scan for Issues" function is the registry cleaner.
Clean function deletes unused files, history, and tracking cookies.
In options:
Set to run when computer starts.
Place cookies you want to save in the save list.
http://www.ccleaner.com/
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How to back up, edit, and restore the registry in Windows XP
http://support.microsoft.com/kb/322756
Safekeeping the Windows XP Registry
http://www.microsoft.com/windowsxp/using/setup/expert/honeycutt_03march17.mspx
How to recover from a corrupted registry that prevents Windows XP from starting
http://support.microsoft.com/kb/307545