Internet speed suddenly slower?

I got a new computer recently because my old one was fried. So I set up the computer and all was well, except for one thing: I noticed that when watching youtube videos, the videos were pausing to buffer a lot and most videos that I watch now have to frequently pause to buffer. This never happened with my old computer, and in the time between my old one getting fried and me getting my new one, I watched videos on my ipod without it needing to stop to buffer. Is my internet being slowed by something, or is there some kind of option to change internet settings to fix this? There was one cord I didn't plug in because when I did, it wouldn't let my computer run normally. It was the PC/USB cord that goes from my router to the computer. Must I plug that in, or does the problem lie elsewhere?