Registry Cleaners

Can somebody recommend a good free registry cleaner? I just ran one and it said I had over 400 problems, but the comments on download.com said some of them may be valid problems that should be kept. Is there any program or way I would know what things would be safe to delete? Thanks for any advise in advance. A friend used one and he said it sped his computer up considerably. I’m just afraid I’d delete a file I need.