Try this

Is it a "Switch" or a "hub". If it's a switch you may need to change the speed on the particular port from 10 to 100mb or even auto would work.

Also, make sure that both of your NICs are set to "obtain an IP address automatically" unless of course you are on a domain that uses static addressing.

I would also try to find out the IP address of the sites you are having difficulty connecting to and ping them from your command prompt. This should show how long it's taking for a round trip. Also, while still in the command prompt, try this: c:\>ipconfig /renew

It could just be that you need to reestablish your connection with the outside world.

I have other suggestions if any of these don't help. It would be helpful for me to know the type of switch you are using.....

try updating your NIC drivers too

hope this helps somewhat,
Mortalkrab