AVG is on par with Norton's. Most anti-viruses are basically the same.
[EDIT: I'm not sure if you completely understand the diffence between defragging and reformating. If you do, ignore this long-winded explaination that makes no-sense. I just needed a break from working on crappy school projects.]
Defragmentation is different from a reformat. I'll modify an analogy I picked up from Wil Wheaton.
Imagine your hard drive is a big book that you enjoy reading. The problem is this book is not bound. Because you're lazy, when you finish a page you just throw it on the ground where-ever you are in the house. So you walk around all day reading and you want to read an old passage, so you go through the house finding those few pages, read them, throw them down, then go back to the big book.
Defragging is like taking a large block of time to go around and pick up every single page and put it back in order. Any rips, tears, smudges, stains, they are still there.
Reformating is like going out and purchasing a brand new book, which means no pages are torn, there aren't any smudges, and the text is perfectly readable. However, this new book doesn't have your little notes in the margin (your preferences) saying what you like and dislike about different passages.
I personally reformat fairly often because I can do it all in a few hours while playing video games, playing pool, etc.
I don't know off the top of my head what else might cause the problems, so I'm going back to my work. Good luck
