ST,
The actions of Avast depend on the way the virus enters your system.
In general, Avast will monitor your computer and scan files upon exection or opening.
For email, the scan is immediate on the content of the message, but the attachments will be scanned if an attempt is made to open them.
Although Eicar is not a "harmful" virus, its' structure is that of a "virus" so Avast will act on it appropriately.
Glad you like the program.
I am puzzled by the fact that you have stated Avast did not pick up Eicar in IE until Standard Shield was set to HIGH. I will forward this on the the Avast team for them.
Take Care,
techie