Is your computer hooked into a router?
If so, reboot your router. It may need to be refreshed.
If you do have a router DEFINATELY check to see if there is a firmware update for it.
That should fix it. I'd also reboot your modem and computer after you install the firmware.
If you don't have a router, I'd contact your Internet provider and let them know. Maybe your modem needs to be updated.
Are you running a firewall software program? If so, it may be interfering and you have to change some settings. Try disabeling the software program, allow your screensaver to kick in, wait a bit and then try to hit the Internet. If you CAN, after it has been disabled, then you have your problem.
If that is the probelm, still do you firmware updates and run all your Windows Updates while you're at it. Now is a good time to clean out your spyware with a program like AdAware from LavaSoft. Reboot everything. Give your machine a tune up!