Hey, Sharp - as others have mentioned, the problem is on your end.
I would start with the basics:
1) Cable is faulty or not properly connected. Swap cables to test this.
2) Hardware configuration on your end is not optimal. Change your TCP/IP settings on your ethernet card to "obtain automatic IP" and the same goes with the DNS server. Put everything on automatic.
3) Your network card has gone bad. I've seen this happen before - namely with cards that are NOT 3Com. Swap network cards if it's not a big hassle.
4) You have spyware, trojan horses, and/or viruses that inhibit you from going online. Unfortunately, most anti-spyware programs and anti-virus programs are useless if you can't even get online to download the latest database files. Run 'msconfig' from the start menu "run...". Click on "start up" tab and unclick all applications which you feel are suspected as malicious software. Note that 'msconfig' is only valid for WinXP based OPs.
5) The port you're connecting to on the router is faulty. Switch ports to test this.
6) Make sure you have access to the router if you are using any MAC filtering. e.g. add your MAC address to the router (using a computer that has access to the router).
7) Right click your network icon in the system tray and click 'repair'. Sometimes Windows will fix the connectivity issue...most times it won't.
I'm sorry about my outlandish behavior - there's too much blood in my alcohol system.