Many people would be surprised at how much space digital videos can take up, and this could easily account for your "missing" space.
While not impossible, I've never heard of a virus that fills a hard drive. I've known of software and movie pirates to drop a trojan onto someone's computer, and use it to serve up pirated software and movies, but they usually like to go after systems on University networks or other really high bandwidth places. Also, with a 40GB drive total, you're not likely worth the effort in the minds of these people.
Simple fact is, 40GB just doesn't go that far these days, and you probably want to be looking for a second drive. You can either get an internal one, or spend a little extra so you don't have to open the case, and get an external. I'd just suggest getting something in the 250-300GB range or more if you're dealing with digital videos.
My Dell computer is less than 3 years old and runs fine, but when I try to upload movies from my video camera it says that I don't have enough space on my hard drive. I checked and it says that of my 40GB I only have 5GB's left. I hardly ever use this computer and there is no way I have filled up all this space. What could be using up all my hard drive space? Is it possible that a virus is using up all my hard drive space?