Hi Dave,
Until someone more knowlegeable answers I'll give it a shot.
1. I keep mine at normal because (a) I have a relatively old and slow computer. However I've never really tried the high setting, so I don't know if it would slow it down much or not (when I had McAfee's it did). (b) I consider myself at fairly low risk for viruses based on how I use my computer and the fact that I've never been infected the entire time I had this computer. However I'm sure if I get complacent and let my guard down too much I will. From what I understand, if you're using Kazaa you're already at a higher risk (but don't take my word for it, I don't know anything about P2P).
Also I suspect the high settings would increase the chance of false positives.
You can always try the high settings and if you don't have any problems then great, if you do have problems go back to normal.
2. I just use the default setting to ask when available because I like knowing what my computer is doing, especially if it's going to take awhile. Also sometimes I like to wait awhile before doing program updates to look at forums like this and see what problems other people may be having with updates before jumping into it myself. So I don't see any advantage, but it's entirely a personal choice.
Do leave the virus def updates automatic (you already know that, but I thought I'd stress it anyway).
3. I have all the programs I'm not using unchecked. However if you think you might ever want to use the others it might be safer to just leave them active. I'm not sure I would remember to go back and activate them if I ever did decide to use the other programs.
My preference is to not have things running I never use. On the other hand I was unable to determine that having all those things running really took up much more resources. I believe VLK has said each provider takes very little physical memory (40k?).
I guess the bottom line is that it's all up to what works best for you. For the most part I just stick with the default settings (I trust the avast folks to make the choices for me).
I hope my less than definitive answers gives you some ideas.
Jarron