A good signal doesn't necessarily mean you're connected.
Are you sure you're properly connected to your wireless network?
First, make sure you get a valid IP, SM and a default gateway values.
After that, ping or try to access your router (aka "Default Gateway").
If communication is successful, try again to access any webpage.
If it fails, and assuming that access to router is successful, ping that website. Make sure you chose a website that can respond to ICMP echo requests (like Google.com).
If all still goes well, ping the IP address to google.
Check if port 80 is open. You can use Telnet command for it.
Should the port be blocked, you can use the netsh command to reset firewall settings.
If you wish to skip all the above tests and go straight to reset firewall settings, you should (no use of quotes):
1. Open CMD by going to Start, Run and type "CMD" followed by OK.
2. Assuming you're running Windows XP, Type: "netsh firewall reset" and press Enter
3. Assuming you're running Windows Vista (God forbid) or later, type the same command, only type word per enter. Meaning: "Netsh" (Enter), "Firewall" (Enter), "Reset" (Enter).
You should get an "Ok".
Restart your computer and re-check communication.
"I'm tired. But then again I'm so hungry I can't sleep. But what's the point of eating if your sleepy?" - Sniff, The Moomins.