Hoov has posted good advice and I would follow it to the letter if I were a novice user...as all of us here are particularly sensitive to rendering instructions for such users so as to even err on the side of caution. It's a good practice to do no harm.
However, in this instance, I can say that those are HTML ASCII characters
. You find them on occasion, due to translating issues as is sometimes encountered with certain languages...computer code languages such as VB (visual basic). I know adobe for one is good for this type of thing. However, these type entries, although alarming at first, can be innocuous. There are some legitimate applications that will produce this behavior. I had an old printer that would show up in certain scan logs showing similar hidden entries in the registry. Being hidden as even more alarming but...it too was legitimate.
In your case, those characters translate to a GUID that relates to Avira. Why CCleaner sees it as unnecessary is a bit of a mystery but then, I haven't seen a scan log entirely, nor have I questioned you about programs installed and such. So...it could be that you either have uninstalled Avira or had it disabled at the time when CCleaner found those entries. On the other hand, it could very well be a needed entry that CCleaner should ignore. With this information I've provided, you can see then that security tools can also produce such entries.
In this case, I would leave it be since it is a harmless entry although possibly unnecessary. I would leave it alone, let's put it that way. It would be my guess that if Avira is still installed, you could use CCleaner to remove that entry but it would more than likely return...instantly. And I say that only because I am familiar with the excellent method by which the authors of Avira have written their program(s). Other, less comparable, tools could be crippled if you used CCleaner to remove such an entry.
Just one more good example why novice folks should stay far away from the registry cleaning software programs unless you know what the entries represent that are produced during an alleged "cleaning" scan of the registry. It's my learned opinion, and shared with my colleagues, that the Windows registry has no need of cleaning...or defragmenting as is another term sometimes used to describe what these tools do.
There are plenty of security tools that remove registry entries which relate to malicious code. This type of "cleaning" is indeed necessary and enough since it's automated so the user has no need of messing about in the registry.