My Avast AV(lastest free version) keeps giving me errors in the event log, it says unable to scan my downloaded files. They are desertcombat mod/AmericasArmy/and Battlefield1942 patches. They are very large(100s of megs per file)zip files and exe files. Does Avast av not scan exe or zip files during download or downloaded? other than this, I am happy with Avast AV.
It depens on your settings in Avast if archives are scanned or not.
If you have enable the scanning of archive files they will be scanned, unless they are in a format Avast doesn’t recognize or if they are password protected.
If you do not use a download manager, so, like Eddy said, it depends on your avast settings. Most probably, you’re using the default level (High) and the files will be scanned.
If you use a download manager, add C:\Program Files\Alwil Software\Avast4\ashQuick.exe to its settings. 
Before I run any .exe or .zip file whcih I have downloaded and saved, I always navigate to it within My Documents, right click on the file and then left click on the Scan with Avast blue icon which should be in the drop down list - then I’m in no doubt as to whether it’s been scanned or not before I run it. 
I’ve found in the past that some trojans can attach themselves to files without my knowing about it so it’s as well always to scan anyway before you run something you’ve saved. Avast would spot them anyway as soon as you tried to run them but better safe than sorry.
You can enable “scan created/modified files” in the standard shield and they will automaticly be scanned
Since this option causes Avast to scan a lot more files, it could have influence on the system performance.
(basicly only on “older” systems)