Bleeping Computer does not
recommend the use of Registry cleaners.
Unless you have a specific problem you're trying to correct, it's best to stay out of the Registry.
Registry cleaners mark entries as bad, depending on what the software creator "thinks", is bad.
That's why no 2 cleaners will find all of the same, "problems".
Registry "problems" actually have very little effect, on the overall performance of the operating system.
Please see this article:XP Fixes Myth #1: Registry Cleaners
If you're not experienced with working in the registry, DO NOT ATTEMPT IT
Improper changes to the registry could render your computer inoperable.
If you do insist on using Registry cleaners, ALWAYS backup the registry BEFORE making any changes.
Instructions, on how to do that, can be found here:How to back up, edit, and restore the registry(I highly recommend, you make a copy of this article.)
what software is the most preferred (and is it acually 100% safe for the registry)?
There are no
guarantees, when it comes to the safety of using, these types of programs.
What might work on one persons computer, causing no problems, might totally wreck, yours.
Personally, I don't think the risk, is worth the questionable benefits.