I think I kind of know what you are asking, I use Avast 5 for protecting my PC against malware infections. The good think about Avast, is that it comes with a web shield, which basically blocks any infected file before it can get to my computer.
I several times clicked on links which was not suppose to, and it simply aborted the connection and informed me that the file I was trying to download contained some sort of trojan or malware.
So even if you are attempting to download a file, that you are not a 100% sure if its infected or not, avast will inform so and prevent it from fully downloading into your computer.
Hope this answers your question.