My friend is running a computer at the same time as I am on the same cable hook up using the proper router. He watches movies. It seems while he is on line at the same time as I that my computer goes quite noticably slower. Is that what happens when another computer runs simultaneously? I have a high speed cable download time of 5 MBPS.

Of course. Your ISP only authorized you so much band-width. If one computer is utilizing the majority of that, your other download speeds will suffer. "So won’t you give this man his wings What a shame To have to beg you to see We’re not all the same What a shame" - Shinedown

Watching movies or doing downloads are bandwidth hogs. If you have cable then that is a half duplex medium that is shared with a lot of other users even in your neighborhood not just the other computer on your network so you are probably getting a lot less than 5 meg . In a half duplex medium you get to like 40% of rated bandwidth things will get really slow.