If you have swapped the cables, cards, and tried different ports on the router, oncluding the wireless approach, I would suspect something software related on the PC.
When you have the trouble, open a command prompt, and do IPCONFIG /ALL to see if you are getting an IP address... Post the IP info up here. Also, if you are getting an IP address that isn't a 169.254.x.x, then trying pinging a webpage from the command prompt....
ping google.com
if that doesn't work, ping Google's IP address: 64.233.187.99
ping 64.233.187.99
report back