The "files identified by large alpha numeric titles" are probably files to do with either your operating system or they are Windows Updates.
"Is Avast more thorough than AVG"
Some will say Avast is and Vice versa. Personally I use neither, I have used Avira for around 4 months and find it excellent, very light on your system resources. From what I could gather Avira tends to score higher on detection than Avast or AVG.