Try this:
Open a command window and type the following command:
NETSH WINSOCK RESET CATALOG
See if that fixes it.
You may want to manually configure your DNS Server locations in your IP config. Make the primary DNS server your default gateway address, and your secondary DNS server as 4.2.2.2. See if that helps.
Another troubleshooting quick-check: at a command prompt, type "ipconfig /flushdns" no quotes. This resets your DNS properties, and forces your internet device (DSL or cable modem) to issue you a new DNS Server.
When this happened to me, I had to try different channels on my wireless router because other rf devices were interfering.
Turn off the Phishing filter in IE7 for faster performance. In the top menu, Tools > Phishing filter.
I have separate programs for firewall, virus, and registry changes.
After tweaking site settings, I think this has to do with blocking persistent cookies, web bugs, and mobile code (Java, etc.).
After letting these updates through, I can get the websites to load.
For me, I delete all un-approved cookies every day, so I don't mind letting these sites through temporarily.
-PJ