How does this work?

[Suicyco247]

If my understanding is correct, Avast only scans a file when you manually scan it or when a file is opened.
 
If a virus or trojan is found when opening a file, is it contained and quarantined before asking what action to take, or have you just unleashed the virus?

I've always made it a habit of manually scanning downloads before opening, but recently forgot. When I opened the program a trojan was detected and promptly deleted. Is this enough to get rid of the trojan, or will I need to follow removal proceedures that I've found elsewhere?

[igor]

If a virus is found & announced when you start an application or when you open a file e.g. in Notepad (technically, when scanning the file "on execute" or "on open"), the virus warning is given before the file is really started/accessed and the access is denied - so you don't have to worry about the infection. The virus is not allowed to launch.

The only exception is scanning the "created/modified files" (disabled by default) - in this case, the warning is given after the file has been actually infected (technically, it's scanned on "close") - but it can't be done in any other way. However, according to what you write, it's not your case.

[Vlk]

And, to complete Igor's description, the file is not quarantined unless you do it yourself. That is, access to the file is blocked (meaning that you're safe) but if you want to quarantine the file (i.e. in case of avast, move it to the Virus Chest), you need to do it yourself...

Vlk