Registry cleaners, unless you are an expert at the registry, are all inherently dangerous and even those considered relatively safe (like CCleaner) can hose your system or make changes advised against by Microsoft.
The only registry utility (actually it is far more than just a registry cleaner) I would recommend to anyone but a registry expert is to use the Windows One-Care free scan which has a registry cleaner incorporated in it.
The difference between this scan and any of the other registry utilities is that One Care is designed specifically for Windows XP by the same folks who wrote XP, not a third party.
Since Microsoft wrote and designed XP, their products designed to work with it are far more preferrable and far less likely to let you make a mistake from which you can't recover or would have tremendous difficulty in doing so.
Windows One Care Free Scan
Go to Windows Live Onecare Free Scan (using Windows Explorer only)
It will say "Get a free PC safety scan"http://safety.live.com/site/en-us/default.htm
Make sure you click "Full Service Scan"
in the middle of the page and not the "Try It Now Free" on the right side.
Allow it to download the Active X components.
Choose "Complete Scan" in the window that opens
Do not click on anything else that offers you a free trial or to sign up if you live in the US.
Allow it to scan - it may take quite, maybe two hours or so depending on how big your hard drive is and how fragmented your registry and drive are.