downloading things from the internet is just fine in most cases. Don't trust downloads from disreputable sites if you are that paranoid. In other words, don't download .doc or .zip files from P2P programs, and don't visit warez sites.
Download.com is a very reputable site, and the Doom download is totally legit, as the source code for the game was released to the public domain years ago. If old games are your thing, then check out
the underdogs, which is a huge archive of abandoned games, including a few of Silicon Knights's classics.
Truth be told though, I have about 20GB of stuff I downloaded from the internet, and I don't even have a virus scanner installed right now. When I did have one, it was one that I got from the internet.
I am basically a trusting fellow, and I RARELY get a virus. When I do, it's usually because I networked mine and my sister's computer, and some of her virulent stuff managed to get into my own. Her computer is like a big breeding ground.
Oh, and you are welcome btw.