What you are explaining shouldn't cause any sort of slow down icons display in explorer, extracted from files, but on the default settings these wouldn't be scanned.
What is the sensitivity setting of the Standard Shield, Normal is the default, High will obviously cause more files to be scanned ?
Folders and files can be excluded but I would first ask what they are and what is in there as excluding folders could leave you vulnerable ?
Exclusions lists:
Standard Shield, Customize, Advanced, Add
Program Settings, Exclusions
I have only one excluded folder and that is where I store my samples and some tools that avast would otherwise detect and I experience none of the issues you mention, I am however using XP.
Yes I believe you should use the email scanner and scan inbound and outbound email, I also thing you should set the Internet Mail sensitivity to High. The avast email scanner may be the first indication that you have undetected malware on your system sending out spam (multiple emails in a period of time). Email is still one of the most common routes of entry for malware.
