Your IP is
blacklisted by several sites. Even though that supposedly only counts for email, sometimes a site will block any access from a blacklisted IP.
That page to which I linked as a "Click here for some suggestions" link; I suggest that you read those suggestions. Also, some of the blacklisting services have a "Details" link, which will give often give some info on how to get off their list.
What has probably happened is that some other client of your ISP has been caught spamming and a block of IPs has been blacklisted. You may need your ISPs help to get their IP(s) off the lists.