Security scan of downloaded files

While downloading any programs, particularly any software, I have seen that the site asks to do a security scan of the files to be downloaded. Sometimes even the full system scan is asked by the site. I dont understand why the system scan is required for downloading any file. As far as I know, if I am downloading the file from any server, its the file that should be scanned and not my system that is required to be scanned to maintain my security.

What exactly does it scans when downloading the files? Is it so much required?

It is not required for an external site to scan your system for downloading any files. Mostly such sites are the crap sites and while scanning your system may retrieve crucial data or may leave a malware in the system. Try not to download anything from such sites that makes it mandatory to scan the system before downloading any file.

Oh, thanks for this information. I will avoid downloading stuffs from such sites.
What about sites that downloads files and then scans the files! Is it also harmful? I think scanning of downloaded files is actually good to be protected from crap download. So is it necessary to scan the setup file of any software that is downloaded or to scan the installed files from that setup files as well?

It is sufficient if you scan the setup file only that is downloaded from the internet. Virus may come together with downloading files, so we want to make the download manager scan the downloaded files automatically before opening it. If you get FlashGet downloaded manager for downloading from internet, it can scan automatically the downloaded files as soon as they are downloaded.

For that, go through this step in FlashGet :

1. Go to the Tools menu, then Options...
2. Then anti-virus. Select the virus scanner to scan the downloaded file(s) by clicking on Browse. You can also check "Scan virus after finishing download(s)". Here Scanning file types can be customized.
3. Alternatively, right-click on any downloaded jobs and choose Virus Scan.

Before opening any file you've downloaded you should right-click the file and select the option Scan for Viruses. A practical solution is to create a folder on your C: drive called 'Downloads' and save everything into that. This gives you quick and easy access to your files and you'll always remember where they are saved.

Once you've put your file into Downloads, you can right click it and scan it for viruses before opening it. NEVER open files from people or sources you're unfamiliar with (whether they come from a website or an e-mail) and always scan them first!

You can also scan files directly on the site online before downloading them so that you can make sure the file is virus free. You can use the Dr.Web plugin, which will add an item to the right-click menu to allow you to scan any web page or download link using their online virus checker.

This addon for firefox can do this trick.
https://addons.mozilla.org/en-US/firefox/addon/938