I ran CCleaner (the "registry" part) and I saw lots of "Unused file extension"s. I unchecked them because I was scare that if they were cleaned, they might not be able to be read by the computer. What happens when they're deleted(or rather, cleaned)?
Unused means that no files on your computer uses it. If a program uses it, it wouldn't be labeled as "Unused." You can always back up your registry andrestore it later. But, I've never heard of it affecting a user.
AJ