Unable to Establish HTTP Session

I’m having a problem that is really becoming frustrating. After rebooting my PC (Windows XP Media Center Edition Service Pack 2) I can initially start an IE browser session and establish an HTTP session. Then at some point something happens and I can no longer establish an HTTP session with any sites. I have no problem establishing HTTPS sessions and when running Windows Network Diagnostic tool it successfully connects to microsoft.com with passive FTP and also with HTTPS but gets an error 12029 connecting to hotmail.com and an error 12002 connecting to microsoft.com with HTTP.

Any assistance that anyone can provide would be greatly appreciated.