I assume if you are burning CD's or DVD's you need to turn it off the same way you would anything else screensavers, the auto start on your CD & DVD players etc. so not to interfere with the burn.
Only on old systems with a non burnproof burner.

Yahoo is webbased mail (unless you pay for it), files stay on the server unless you choose to download/save a copy of the message or attachment to your local system. Webbased mail does not need configuration by Avast. If you download the mail/attachment, the on-access scanner can take care of it.

Does the Avast on access scanner have to be visable down in the right hand corner to be working ? or like other things if you turn it off in msconfig is it still working ?
Firts part (the blue balls): You can merge them or hide them with RejZor's AEC tool.

Second part (disabling in msconfig):
Disabling will make Avast (or at least part of it) stop working.
Better is to go to the control panel > add/remove programs > Avast > change/repair > remove all providers from Avast you don’t need/want.

Suggestion: Hava a look at Avast’s help, a lot is explained there in simple words.

Avast 4.5 (beta) is even better when it comes to mail handling. (Upgrade Instructions)