Virus scanners reacting to downloaded files

It happens sometimes and since now I just shrugged it off, but it's kinda bothering me: Sometimes my virus scanner (I'm using avast!) alarms me when I'm trying to unzip files I downloaded here. I think the risk of there actually being viruses is pretty small on account of the number of people downloading here, someone would've probably warned the others long before I started the download, but how big is the risk really? Is this just my scanner being oversensitive or can people catch viruses here?