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I have windows 7 and use Avast free antivirus; I wonder if virus are detected by scanning the installer or setup file before starting to install, so that I am safe before the installatoin starts.

Thank you.

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Any executable can be malicious, and a antivirus scan never means "its safe", only that "I don't know if its safe, but its not like the other unsafe things I've heard of".

so yes, viruses may be detected in an installer, detected based on what they do as they are installing or afterward when they run, or not detected at all.

merely downloading a file is not usually sufficient to cause malware to activate. it usually must be run to do anything bad.

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Any real time scanner would alert you of a threat as soon as it was downloaded. You can try right clicking the file and scanning with your AV if it is available or opening the interface and looking for options

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As long as it's detected, but it's not always the case. – gronostaj Jun 17 '13 at 17:09
That is another issue entirely. Testing in a VM would be the safest solution in this case. – Scandalist Jun 17 '13 at 17:18

Remember to always download software from reliable sources. While your anti-malware app should detect any malware in the setup executable (no app is perfect of course, and new malware pops up daily), it might not be able to initially detect infected files inside the executable. This can happen if the program was repacked by someone (often the case with cracked programs or pre-cracked releases) and the anti-malware app cannot unpack whatever compressed format the setup executable uses. Hopefully though the real-time scanner should pick up the infected files as soon as the installer extracts them, or at the very least when it tries to execute them.

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