You're probably infected, some sort of Internet clap. Your IP address is likely the cause of 'too much traffic' and so something somewhere (perhaps a DNS Server somewhere or a proxy) has decided you are a threat and your traffic is being dropped when you try and go there (or via there - maybe something at your ISP). Likely cos they have determined that you are trying a Denial of Service attack or something.
Helpdesk monkeys are mongs on the whole so might not work it out.
If you've reinstalled Windows and Firefox etc, then it's either re-infecting you (so check your perimeter security - do you have a firewall built into your router, personal firewall etc or not) or pop-up blocker, spam filter, adaware etc etc. If not, get it.
Actually, just googled this before posting, and it could also be any number of other things, a dodgy proxy, a particular site objecting to you accessing it from different locations (some sites set a cookie which identifies you, you access from somewhere else and it thinks you are an intruder etc). Looks like it can also occur when your ISP is using a Proxy server - although this then implies that you don't have a real IP address, and are part of an internal network the ISP is using?
Ben S may know - as I think he's an internet ISP type person.