Slow internet on one computer makes the internet slow on others?

[Mary]

Whenever I'm on this one computer, the internet is slow. Sometimes it's normal, but it's usually slow. Now this doesn't happen on other computers, just the one. My biggest problem is that when the internet runs slow on the one computer, it makes the entire wifi slow. I can't use the internet on an iPod or another computer because now it's slow because of the one computer. It only happens on this computer. If I don't use this computer for like a day, the wifi is fine. But if I use the computer, wifi will go in between slow and normal. I don't understand why this happens. I hope I explained this well enough. It just frustrates me.