There isn't any way to be 100% sure what a file contains before you d/l it, but if your careful and check the file sizes and usa a good virus scanner you should be o.k. For more info on viruses go here: 
MAJOR TOPICS for Open Discussion HELP 
One thing you can do before you d/l something is to look it up  using Bitzi. Learn to use it here: 
How to use Bitzi Web Lookup ...    Quote:
   | Also when people are uploading from me are they able to hit me with viruses? | 
 
  As far as I know it's not possible, they're uploading from you not downloading to you. Good luck

. gtg