Thanks so much for the detailed replies that explain things I asked about. Now I understand more.
By the way, I started this thread by asking if I needed to disable avast when I ran a Defrag. At that time when I ran a Defrag "Analyze," it said the system needed to be defragged.
But I waited to defrag when I found out that I should delete MSE so as not to have 2 AVs on the pc. So after asking a million questions and getting such wonderful help from all of you, I finally DL'd CCleaner, uninstalled MSE, and ran the uninstaller tool. Then I used CCleaner for general cleaning (I have not used it for registry cleaning, as I'm too hesitant about that yet. I may do so in the future if I feel more confident about it. But as bob 3160 says, the safest thing to do is not to clean the registry when you don't know what you're doing.)
Anyway, I then ran another "Analyze" to see about defragging, and it now says the system does not need to be defragged!! So does that mean that doing all of the above realligned files on the disk enough to change the need to defrag???
Also, some day I may switch from Spybot to SuperAntiSpyware as advised. But I need to move onto other activities now, so that will have to wait.