Can I check a download for viruses before I download it?

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I'd like to prevent viruses from ever reaching my machine - can I test them before they're downloaded? And if so, how?

In a word, no.

But there are definitely precautions you can, and should take, that will allow you to deal with downloads safely.

The bottom line is that a download must be on your machine (or a machine on your local network) before you can check it for viruses. So, with that in mind, follow these steps to download safely:

• Download only from sites you trust. I know this is kind of vague (how do I know what sites to trust?), but in general that means that downloads from major hardware and software vendors, major shareware site and other reputable companies. If you're not sure, Google them to see if other people are experiencing trouble with downloads from that site.
• Always download, and never run any download. For example many programs for Windows are provided as a downloadable ".EXE", or executable file. The default action for a ".EXE" file is to run it ... meaning that the program contained in the .EXE would be allowed to run and do whatever it might want. Instead, download the file to a directory on your computer first, so that you can take the next step.
• Scan the download for viruses before installing or running it. Most anti-virus packages have the ability to scan a single file, or perhaps a single directory. Use that to point at the file or the directory you've downloaded your file into, and do the scan. Make sure your anti-virus scanner's data base is up to date.. Consider an anti-spyware scan as well.
• Assuming your anti-virus software reported no problems, install, run or otherwise use the download.
• If you have reason to be paranoid, it often makes sense to re-run the anti-virus scan and the anti-spyware scan after you've installed your download.

So what if your download shows up as being infected? First, delete the copy you just downloaded immediately, so that it doesn't get run by accident. If you can, see if you can find the same download from another source - sometimes malware is present only in some of the downloads of a particular piece of software.

If you can't find a clean download - please don't fall to the temptation of installing it anyway - it's not worth the risk unless you really know what you're doing. Contact the supplier or manufacturer of whatever it is you're downloading and report the issue to them. If they're at all reputable, they'll deal with the issue quickly.

Related:

• Ask Leo! - How do I keep my computer safe on the internet?

• Ask Leo! - Recommendation: Anti-Virus Software

• Ask Leo! - When do I actually need to run a virus scan?