CCleaner is what we might call a "light" registry cleaner, or even "safe".
It really only goes about detecting entries that are for-sure unnessasary and/or dead.
The number of problems I've seen fixed by a CCleaner registry clean are far outweighed by the number of problems I've seen it cause. Wha.t seals the deal for me is that the problems it fixes are usually something innocuous , like a message or "missing file" error on startup, whereas the problems range from Office not working entirely, Photoshop suddenly being unable to read and write to a network drive, and even entirely unbootable systems. Most commonly, it can often break network-based setups if all the network drives aren't mapped, both because the drive specification would be invalid (leading the cleaner to assume the file doesn't exist and that the entry is invalid) or because the cleaner program is too stupid to understand the need to use the Wide character functions and prepend paths with /?// and thus entries longer than 240 characters get truncated, resulting in that the once again not finding said file and marking it as invalid. This doesn't really mix well with it's disregard for UNC at all, which are disregarded as invalid because they don't have a drive letter.
It is not nearly 100% safe, though it is definitely less aggressive than a lot of registry cleaners. there are regular topics titled "CCleaner killed my computer!" and similar in the Piriform forums. Since it's not 100% safe, any recommendation to use it's Registry cleaner component is taking a gamble with another person's machine. The Disk cleaning component is fine, if only because it pretty much just does things you could already do at once- run disk cleanup, clear Browser cache's, etcetera.
The only way to use it the Registry Cleaning part "safely" is to inspect the results, but if you are able to do that and know what they mean, you probably wouldn't be using the cleaner in the first place.
I always select "Yes" when prompted for backup, but I have never once had to use a backup made by CCleaner because I have never had it go wrong.
CCleaner support says on its page asking if CCleaner is safe:
"Yes. CCleaner is absolutely safe to use.
CCleaner only removes files that we know that you no longer need. It has been downloaded nearly 1 billion times since it was first released in 2004 making it one of the world's most popular software titles." - Piriform ( http://support.piriform.com/entries/21184736-is-ccleaner-safe-to-use )
Although download numbers themselves do not say much, that is an incredible number. Lots of technicians and home users trust CCleaner.
Aren't they pulling out an ad populum fallacy here? Instead of actually providing any strong reasoning as to why CCleaner is safe, they just appeal to it's popularity as proof that it's safe. That's begging the question.
Windows 7 defragmenter, but what I do observe is that Defraggler supports scheduling which is great for Windows XP, ME, 2000, and 98
Hopefully it just provides a UI from which it creates jobs using the "Scheduled Tasks"/"Task Scheduler" component present on all those systems. (as opposed to one of those dumb reinvent the idea of scheduling things that requires you to run some program in the background to launch the defragmentor). most of them do, but this doesn't really add functionality as much as make existing functionality present in the OS available.
Indeed, in many cases, having more than one AV program will quickly render your computer unusable until one is removed. (AV scanners integrate themselves in such a way that they tend to be incompatible with having more than one program running).
The problem with multiple AV scanners in the background is that, from the point of view of Anti-Virus A, the actions of Anti-Virus B could easily fit the profile of malware- Even the act of 'cleaning' or moving a file to a quarantine or whatever they do these days could be interpreted by a separate AV as malicious behaviour. All the other AV sees is some other program hooking and changing process memory, injecting executable code into processes, hooking system import tables to redirect functions like CreateFile/RegOpenKey/etc. to point to it's own functions. All of these are the hallmarks of viruses and malware, in the last case, specifically of rootkits. So what will happen is the second AV sees this, and goes "OMG there is a virus" and prompts the user. The poor user, thinking they are infected, and being presented with the typical over the top prompt screens AVs use, will often choose to 'clean' the infected file. Of course, what makes this worse is typically both AV programs see each other at the same time, so every program load will sometimes show an infection prompt from both AVs simultaneously, misleading the user into thinking that the file is infected,(why else would both the AVs show a prompt right after they ran it?) when the reason for the detection was in fact the operation of both AVs from the reference frame of the other.
What usually ensues when the user clicks to "clean" is a battle. First, the "cleaning" AV needs to try to unhook the functions the other AV is using safely. This just makes things worse, since AVs are usually built to detect this since it is a method that could easily be deployed by actual malware to circumvent AV programs. So now it's alarm bells are going off. So the user tells <it> to clean, and now we have both AV softwares duking it out trying to dehook each other and presenting the user with dialogs.
In the meantime, said user is getting nothing useful done and is probably stressing out thinking they have a major malware infection when in fact all they have is two AV programs in a collosal battle for domination, each one running as though the other were an evil piece of malware that is fighting back.