(I'm getting auto-moderated, so I'm splitting this post up.)
Second, you should always evaluate WHO is giving you the advise, before you even consider the advise itself. I am constantly having to deal with people that have made their bad situations even worse because they assumed the person that sounded like they knew what they were talking about actually knew what they were talking about. Most often, they have what I would call "negative knowledge". They know so little it can only be described in negative numbers. In order to educate these people, you have to teach them how to get all the way back up to "0". They hate hearing this, and get pretty grumpy when I tell them, so I've quit. Even when I win the technical arguement, what I get in the end is the opportunity to put a lot of time and effort into helping someone else fix their computer. So I don't waste my time.
But here's a few "gems", if you will.
Many people assume that the way to make a computer go faster is to install more software. More "accellerators" and "optimizers" etc... It's like putting rocks into the trunk and expecting the car to go faster. Computers go faster the less they have to do. It's easier to sell someone a magic-bullet software than it is to teach them how to make their system go faster. And people are lazy and stupid. They want to believe that some magic software is going to solve all their problems, and they also get kind of grumpy when you tell them different.
For example, Registry Cleaners are PURE BS, and anyone that says different is ignorant, stupid, dishonest or some combination of the three. It sort of "feels" like it would be a good idea to "clean" a registry, because from a non-technical perspective you might buy-in to the idea that a registry can be "dirty". A registry is not a toilet, and it can neither be dirty or clean. It's several files (5, exactly) that have a bunch of information in them that is necessary to run the computer. Some people would have you believe that having unnecessary information in these files might somehow slow your computer down. Yet, there is not one shred of evidence anywhere to prove this. Nowhere. No studies, no proof, not even a credible expert (like a Microsoft Engineer) to come out and publically say that a "cleaned" registry is going to make the computer run faster.
People confuse the difference between "unnecessary" registry entries and "bad" registry entries. The bad ones are bad for definable reasons, and the "unnecessary" entries allow the retarded to believe they know something that they do not. Oh yeah, and it makes software developers money, too. Even Microsoft has a "registry cleaner" now. (But no one from Microsoft can explain how removing unnecessary entries speeds-up a system.)
However, when the software deletes a GOOD registry entry, guess what? Your computer either slows down (errors on boot, program launch, etc...) or it stops working completely. So, registry cleaners = no benefit, and significant risk of causing problems.
Real Computer Techs know this. It's common knowledge. So, when I see someone advising the use of a registry cleaner, I already have some idea of what their level of expertise is.
"But-but-but I ran CCCleaner and the problem that I had went away."
Maybe. And maybe you could have found that bad registry entry manually. And also, while CCCleaner was "fixing" the problem you knew about, it also created several more that you DON'T know about, until one day you discover you don't have any System Restore files anymore, or your Administrator Account has disappeared, or your computer won't boot into Safe Mode anymore, or Windows Update won't install KB187823A, or the permissions on some Registry Entries won't allow you to uninstall Norton Anti-Virus, or (there's thousands and thousands of ways a computer can be broken, and the User doesn't know about it).
Point? Most Users aren't qualified to have an opinion on whether their computer works or does not. Most people don't even know that they are infected, when they are. People that can't tell when they are infected do not have the credibility to offer opinions on how to make a computer work faster. The have negative knowledge, and will require a significant amount of time & energy just to get them up to level "0", and you don't get that for free.
So, the best advice you can be given is, go to the place where people that have a passion for fixing computers hang out, and don't take advice from people that don't know anything.