Problem getting ethernet connection on new computer...

Hello my name is Jordan. I just purchased a new computer and i'm trying to get an ethernet connection going on my cable modem. I've tried going straight from the modem to the computer but it won't recognize the cable or network, nor does it recognize it when I try to go through the router I have. It does pick up a wireless network connection from the router, but it only goes at dial up speeds. When I restart the computer I noticed it does start at about 600K and then quickly wines down to 60k within a minute or so.

And if it helps you help me, ill add that under system network devices the Realtek network device is gone and all I see is the Broadcom wireless card device. And under network connections there used to be 2, the Broadcom Wireless one and the Realtek Local Area Network one, but for some reason the Local Area Network one isn't there anymore.  I did reinstall the drivers for the Realtek device but it didn't do anything. Thanks in advance for any help, cuz im clueless on what to do and id call my ISP but this seems like a computer problem and not a service problem, plus they usually give me the run around and then charge me for a service visit anyways.