Viruses
 

you should know the file your are looking for and its size.

infected files usually are zip files, under 1MB (1000KB), and there are at least 7 identical files in the search criteria.

I also recomend getting McAfee VirusScan. this program has a thing called Active Sheild which protects your computer. Like if you download a infected file, like 5 seconds after your unzip/expand it, Active Sheild will have automatically detected it, and deleted the infected file for you. Spread the word and stay away from viruses :D