Bring up the cmd window and do three pings :
ping 192.168.2.1 (sub your router's ip address if this is not it)
ping 64.233.167.104
ping
http://www.google.com
If the first one returns timeouts, the fault is local, either the NIC, cable or router. Up front, I'm thinking probably not because you said the appropriate lights are on, but it's still a possibility.
If the second returns timeouts, your connection may be a bit shaky. Call up your ISP and have them check your synch rate, they'll be able to tell if there's any physical faults on the line.
If the third one returns "Unable to resolve host : http://www.google.com/" then the issue is DNS. You can also try switching your TFP bookmark to
http://209.50.251.11/ and see if it clears up on that site, that'll clinch it. If that's the case, switch DNS servers (I use 4.2.2.1 primary and 4.2.2.2 secondary) see if that helps. If not, the problem's local and requires a regfix. I know the ISP I work for will walk you through it, but that will vary from one to the next. Messing around with your registry can be both daunting and dangerous, so if this is the result you get it may be time to call in the big guns (ie your local guru).
EDIT - Excuse my tired and insensible posting from last night in which I assumed you're using DSL service. If it's cable they can't check your synch rate because you won't have one, but the rest applies.