Tech Help...please...

[thepascalblaze]

My father-in-law's computer has an almost full hard drive. It's 80 gigs. It's less than two years old and he only has put a couple of gigs of stuff on it himself. I was there for Easter yesterday and I couldn't find the files that take up so much space. Are there viruses that fill a hard drive with garbage data?.. or perhaps make a cpu "think" its hard drive is nearly full? Any help would be greatly appreciated. I am of limited knowledge so, please, keep answers simple, simple, simple...and type s-l-o-w-l-y.