avast doesn't block it isn't a firewall, it scans and alerts to infection. But, avast doesn't block unless you have added the domain to the Web Shield, URL Blocking section ?
I would suggest you check your HOSTS file to see if there isn't some redirect in there, but it sounds a bit more than HOSTs file blocking.
I would have said check your firewall but the XP excuse for a firewall doesn't have any outbound checking.
What other security based software do you have that might, e.g. anti-spyware, security applications, etc. ?