catching viruses without scanning

[n4wwl]

I am running avast! on 4 home computers.  Every couple of weeks, I do a scan for viruses and avast usually catches a few on several machines.  

My question is:  Why do I need to do a full virus scan to catch these viruses?  I thought avast! would notify me as soon as I pick up a virus from web surfing or e-mails, as long as the computers are on.

Thanks.

[Lisandro]

Why do I need to do a full virus scan to catch these viruses?

Are your residents providers on all the time? If not...
Other possibilities:
a) new virus definitions (updates) detect virus already installed in your computer (?)
b) you are not seing 'infected files' being reported but some other kind of files (password protected, access denied and so on...)  

[n4wwl]

Yes, my resident providers are always on.  The virus definitions updates occur regularly with no problem.  The infected files are always new and not password protected files previously resident.  It just seems strange.

Thanks for helping me troubleshoot this.

[Lisandro]

The infected files are always new... It just seems strange.

It won't be strange if we are talking about archive files (such .zip, .rar).

They will be downloaded and only scanned if you ask to:

1) from download manager (ashquick.exe generally without parameters, or %d or "%f")
2) from the Standard Shield configuring the extension list:

Scan files on open
WS?,VBS,VBE,JS,JSE,HTA,WSF,WSH,SHS,SHB,HTM*

Scan created/modified files
ACE,ARC,ARJ,BZIP2,CAB,COM,GZIP,PST,RAR,TAR,ZIP,ZOO,ECE

For Home version, you must edit deftasks.xml file in the avast folder and add the following line

<STANDARD--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ</STANDARD--ScanPackers>

after the line
<STANDARD--InvalidEntry>1</STANDARD--InvalidEntry>

[n4wwl]

The infected files it catches are usually in the user cookie directory or in the Windows system directory.  That suggets to me that the infections are being allowed through as a result of web browsing and are probably not archived files.

[Vlk]

Unless you set the sensitivity of the on-access scanner to High, those beasts will not be properly detected in real-time. Even if you do, some of the files may be missed (depends on their filenames). You can enable full scanning (i.e. scanning of all files on write) by using the Custom level (on the second Standard Shield tab, Scan Created/Modified Files, put the asterisk (*) symbol to the extensions box) but there's a big overhead - every file will be scanned then. But it's not a bad choice if your hardware can afford it...

Thanks
Vlk