#1. The avast! 4.x Home/Pro would be the correct forum for avast related questions/problems, this forum the Viruses and Worms forums is specifically for that issue.
#2. As that section is aptly named Troubleshooting, if you aren’t experiencing any trouble, leave well alone. If you are experiencing any problems then yell out and we will advise and if needs be suggest changing any of these settings.
#3. It doesn’t do any DOS type scan before boot-time (like the one after installation) unless you schedule it to happen.
#4. The rootkit function is an integral part of avast and runs 8 minutes after boot and as a part of on-demand scans (Standard and Thorough sensitivity only) from the Simple User Interface.
The default setting in ‘all’ cases provide the best balance between performance and protection. So until you get more familiar with avast I would leave ‘all’ settings as they are, not just these. Changing settings without being familiar with what they might do could have unforeseen effects.
Stick around and browse the forums, especially the sticky topics at the top of each of the forums, not to mention the avast help file. They provide a wealth of information to help you get the best from avast.
If it isn’t broken don’t fix it, these options are specifically for ‘troubleshooting’ specific problems, which aren’t being reported by the OP dave1510.