A few recommended articles to read on registry cleaners:
Registry cleaners are not supported by Microsoft
Microsoft support policy for the use of registry cleaning utilities
An often posted and quoted article (Ed no longer hosts the site but did mention again on Twitter), Ed Bott's Why I don't use registry cleaners
miekiemoes' Blog Registry Cleaners and System Tweaking Tools
Registry cleaner - From Wikipedia
Are Registry Cleaners Safe to Use?