Thanks for the replies!
Only Avast was running when I did this tests. I uninstalled all antivirus software and installed a clean Avast Free (unless it remembers previous settings, but I don't remember messing the settings before).
Now, I've just booted the PC, waited for everything to be started, and I was able to run eicar.com (in my desktop) with no warnings.
Then I decided to check the settings. File System Shield options were:
- "Scan when executing" screen: all checked
- "Scan when opening" screen: just "Scan Documents when opening" checked
Now, if I check "Scan all files" in the last screen, voilà: I receive an warning from Avast and eicar.com doesn't run. But if I disable this option, I can run eicar.com again (I left half a dozen in my desktop for testing).
Is this the expected Avast free behaviour? It appears Avast free thinks eicar.com is a document, not a program (and a document type it shouldn't verify). What are the default settings?