I noticed that I have very high hdd activity while I am not at my PC. When I am leaving my computer (not shutting down), after the screen turns off, the hdd starts to work immediately. What happens here? Is it some kind of defragmentation?
Windows 7 Defragmentation Schedule default runs at 1:00 AM every weekly.
Windows 7 Search runs in background in a low priority and costs little resource with default configuration. But if you have used PC for a long time, you should care about Internet Explorer caching store.
I've never used Avast, so I guess you have to check your Avast Scanning Schedule or Scanning Rule.