If you see an error similar to the one in the subject line (also seen below), it is likely that this is because the flv file has no data in it (0 bytes).
This problem is most often seen on Youtube and is caused by YT blocking the download of the file.  Often, once such an error is encountered

ALL subsequent downloads from Youtube will also be blocked.

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Another way to confirm this problems is by viewing the progress bar (shown in the photo below) move as the file downloads. 
If the animation showing a file flying from the globe to the folder plays, but the progress indicator doesn’t move, this is a sure sign
you have a problem with a download being blocked. 

To solve this problem you need to simple clear the cookies and Temporary internet files in your browser.

After doing this open Video Surgeon and using IE play a video, allow the sniffer to find it, and then go ahead and start the download.
You should now see the blue progress indicator moving – which is a sign that the problem has been fixed. In addition to clearing the
cookies and cache, you may also need to set firewall exceptions if this problem happens again.  Search the knowledgebase to find
the article on setting firewall exceptions.