I suspect a problem with firewall software such as ZoneAlarm, which had a known incompatibility with a Windows update issued in July: http://support.microsoft.com/kb/951748
Bottom line: If you are running ZoneAlarm or any other 3rd party firewall, internet security or other protection software (including anti-malware software), *uninstall* or disable it, restart the system, then see if you can get online.
Unfortunately by over-protecting the system these types of programs end up causing more problems than they solve, which is why I don't use or recommend them in general. When troubleshooting any sort of problem like this (or any problem with Windows in general), the *first* step I take is to disable or uninstall any "protection" software to see if it might be the cause.
Let me know if this doesn't help, in that case there are some other things you can try. Scott.