Answering Vlk's questions for my particular circumstances...
1: 64 bit build, English
2: Yes, it's related to Web Shield; I tested this by repeating my visit to the sites that fail and disabling *only* the Web Shield provider- as soon as I did so, the site loaded; so long as Web Shield is running, it fails to load. Pausing does not correct the problem (I.E.: the effected web pages will only successfully load if I terminate the provider)
3. I tried manually configuring the WebShield proxy as described. It appears to make no difference (I.E.: with the proxy in place and WebShield active, access to key websites such as
www.microsoft.com fails; as soon as I disable WebShield, the page loads successfully)
Thank you for your interest and attention regarding this problem!