Modem and internet

Hello,

My internet works fine when my computer is directly plugged into my modem, but it becomes laggy/won't work at all when I add my router to the set. My computer is not wireless but I have a laptop I use that requires the router instead. At times when my computer is plugged into the router however, the internet is fast and pages load perfectly. But then there are times when it does the opposite unless I plug it back to the modem instead. My computer states that there are no problems with connectivity.

I use windows 7. Any help would be much appreciated as I know nothing about computers.
Thanks in advance!