I might be thinking simplistically, but have you tried dialup from this machine? DSL providers that also provide dialup access normally use different IP ranges for DSL and dialup. There is a possiblity someone in the ISPs DSL IP range has done something to get the whole "domain" kicked from the site. If dialup works on the machine you will rule out the machine being the issue, and it will point towards the IP you are getting from you ISP (the most likely case). The problem with this senario is you have very very little to say in the matter, mainly because the company is allowed to block anyone they wish from using thier resources, you might contact the company that runs the site to 'appeal' the issue if it is an IP based block.

Cheeseball