It could be that your anti-virus is set to scan files as they download. So the scanning does it by scanning pieces of data as if they were executable files fooling the virus into thinking the program is running which a virus will activate itself by replicating itself in the downloadable file- thus beiing found by the anti-virus software.

What was the question again- lol!!

Try disabling the feature in your antivirus program - you can scan that particular file after a download by right clicking on the program and choose scan from the drop down list.

The same problem happens when you upload email attachments or when your downloading attachments. It is more safer to scan while a download but you pay the price in the time it takes.