Depends. If you have more than 256Mb of RAM, you probably can configure avast to scan all files (open/created/modified) without a lack of resources.
If you slow down your system too much, please, write at least, the following lines:

Scan files on open:
WS?,VBS,VBE,JS,JSE,HTA,WSF,WSH,SHS,SHB,HTM*,PIF,EXE,PIF,COM,SCR

Scan created/modified files:
ACE,ARC,ARJ,BZIP2,CAB,COM,ECE,EXE,GZ,GZIP,LHarc,MIME,PST,RAR,TAR,WinExec,ZIP,ZOO

What do you mean? ashMaiSv.exe is the executable file of avast Mail Service. You must have it running to scan your emails.

I suggest you close your preview pane in IncrediMail (the behavior is the same on Outlook Express). Properly configurated means:

  1. all windows patches applied (specially SP1 and those ones related to OE)
  2. avast sensibility set to ‘High’
  3. good firewall

If you are at the Internet Mail settings yes, this applies to you.
But it’s better to change the Standard Shield sentivity to High too.

Do you have any other question? ;D