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>