Why Is My Computer Running Slow

Looking at my computer I’m building I look at the hard drive and 1TB seems to be a lot of space. Never ever have I filled up a hard drive before. I heard somewhere that if you have too much space on your hard drive it slows your computer. Is this true?

I share my computer with my boyfriend and it is very slow. He has 100 songs on his playlist posted on my space. Could this cause the computer to be slow?