What firewall are you running?
What browser are you using?
If it’s IE and it’s not working, I would try downloading and installing Firefox. Far superior browser than IE.
If nothing else works, what I would suggest is if you rarely shut your computer down, try doing a cold reboot… shutting all the way down and then starting it back up. If that doesn’t work, it might be time for a reinstall of windows… sometimes when you haven’t had a reinstall for a while, the browsers and other programs start to do odd things. If you can’t reinstall windows, you might try booting into safe-mode and running a disk defrag.